diff --git a/filtermail/Cargo.lock b/filtermail/Cargo.lock
index c5739c96..1737ac71 100644
--- a/filtermail/Cargo.lock
+++ b/filtermail/Cargo.lock
@@ -2,18 +2,6 @@
 # It is not intended for manual editing.
 version = 4
 
-[[package]]
-name = "ahash"
-version = "0.8.12"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5a15f179cd60c4584b8a8c596927aadc462e27f2ca70c04e0071964a73ba7a75"
-dependencies = [
- "cfg-if",
- "once_cell",
- "version_check",
- "zerocopy",
-]
-
 [[package]]
 name = "aho-corasick"
 version = "1.1.4"
@@ -79,15 +67,6 @@ dependencies = [
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 ]
 
-[[package]]
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-version = "0.2.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f0c269894b6fe5e9d7ada0cf69b5bf847ff35bc25fc271f08e1d080fce80339a"
-dependencies = [
- "object",
-]
-
 [[package]]
 name = "async-trait"
 version = "0.1.89"
@@ -123,16 +102,6 @@ version = "1.11.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b35204fbdc0b3f4446b89fc1ac2cf84a8a68971995d0bf2e925ec7cd960f9cb3"
 
-[[package]]
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-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "cd4932aefd12402b36c60956a4fe0035421f544799057659ff86f923657aada3"
-dependencies = [
- "find-msvc-tools",
- "shlex",
-]
-
 [[package]]
 name = "cfg-if"
 version = "1.0.4"
@@ -149,16 +118,6 @@ dependencies = [
  "encoding_rs",
 ]
 
-[[package]]
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-version = "0.9.3"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8eebd66744a15ded14960ab4ccdbfb51ad3b81f51f3f04a80adac98c985396c9"
-dependencies = [
- "hashbrown 0.14.5",
- "stacker",
-]
-
 [[package]]
 name = "colorchoice"
 version = "1.0.4"
@@ -191,23 +150,6 @@ version = "2.10.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d7a1e2f27636f116493b8b860f5546edb47c8d8f8ea73e1d2a20be88e28d1fea"
 
-[[package]]
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-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "97369cbbc041bc366949bc74d34658d6cda5621039731c6310521892a3a20ae0"
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- "quote",
- "syn",
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-
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-checksum = "e079f19b08ca6239f47f8ba8509c11cf3ea30095831f7fed61441475edd8c449"
-
 [[package]]
 name = "encoding_rs"
 version = "0.8.35"
@@ -264,7 +206,6 @@ dependencies = [
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  "env_logger",
  "governor",
- "lettre",
  "log",
  "mailparse",
  "rstest",
@@ -275,39 +216,18 @@ dependencies = [
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 ]
 
-[[package]]
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-version = "0.1.7"
-source = "registry+https://github.com/rust-lang/crates.io-index"
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-
 [[package]]
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-
 [[package]]
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-
 [[package]]
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@@ -344,11 +264,9 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
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@@ -402,10 +320,6 @@ name = "hashbrown"
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 [[package]]
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@@ -418,108 +332,6 @@ dependencies = [
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@@ -570,38 +382,12 @@ dependencies = [
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-
 [[package]]
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@@ -645,30 +431,12 @@ dependencies = [
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-[[package]]
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-version = "8.0.0"
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-checksum = "df9761775871bdef83bee530e60050f7e54b1105350d6884eb0fb4f46c2f9405"
-dependencies = [
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-
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-dependencies = [
- "memchr",
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-
 [[package]]
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@@ -704,12 +472,6 @@ dependencies = [
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-[[package]]
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-
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 version = "0.2.16"
@@ -737,15 +499,6 @@ dependencies = [
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-[[package]]
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-dependencies = [
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-
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@@ -773,16 +526,6 @@ dependencies = [
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-version = "0.1.28"
-source = "registry+https://github.com/rust-lang/crates.io-index"
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-dependencies = [
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-
 [[package]]
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@@ -997,12 +740,6 @@ dependencies = [
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-source = "registry+https://github.com/rust-lang/crates.io-index"
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-
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@@ -1044,25 +781,6 @@ dependencies = [
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- "psm",
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@@ -1074,17 +792,6 @@ dependencies = [
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-dependencies = [
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-
 [[package]]
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@@ -1111,16 +818,6 @@ dependencies = [
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 ]
 
-[[package]]
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-source = "registry+https://github.com/rust-lang/crates.io-index"
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-dependencies = [
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- "zerovec",
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-
 [[package]]
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@@ -1185,36 +882,12 @@ version = "1.0.22"
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- "serde",
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-
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-
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-
 [[package]]
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@@ -1323,22 +996,13 @@ version = "0.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f0805222e57f7521d6a62e36fa9163bc891acd422f971defe97d64e70d0a4fe5"
 
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-dependencies = [
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-
 [[package]]
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 checksum = "f2f500e4d28234f72040990ec9d39e3a6b950f9f22d3dba18416c35882612bcb"
 dependencies = [
- "windows-targets 0.53.5",
+ "windows-targets",
 ]
 
 [[package]]
@@ -1350,22 +1014,6 @@ dependencies = [
  "windows-link",
 ]
 
-[[package]]
-name = "windows-targets"
-version = "0.52.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
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- "windows_i686_msvc 0.52.6",
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- "windows_x86_64_msvc 0.52.6",
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-
 [[package]]
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@@ -1373,106 +1021,58 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "4945f9f551b88e0d65f3db0bc25c33b8acea4d9e41163edf90dcd0b19f9069f3"
 dependencies = [
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- "windows_i686_msvc 0.53.1",
- "windows_x86_64_gnu 0.53.1",
- "windows_x86_64_gnullvm 0.53.1",
- "windows_x86_64_msvc 0.53.1",
+ "windows_aarch64_gnullvm",
+ "windows_aarch64_msvc",
+ "windows_i686_gnu",
+ "windows_i686_gnullvm",
+ "windows_i686_msvc",
+ "windows_x86_64_gnu",
+ "windows_x86_64_gnullvm",
+ "windows_x86_64_msvc",
 ]
 
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 name = "windows_i686_msvc"
 version = "0.53.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1e7ac75179f18232fe9c285163565a57ef8d3c89254a30685b57d83a38d326c2"
 
-[[package]]
-name = "windows_x86_64_gnu"
-version = "0.52.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "147a5c80aabfbf0c7d901cb5895d1de30ef2907eb21fbbab29ca94c5b08b1a78"
-
 [[package]]
 name = "windows_x86_64_gnu"
 version = "0.53.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9c3842cdd74a865a8066ab39c8a7a473c0778a3f29370b5fd6b4b9aa7df4a499"
 
-[[package]]
-name = "windows_x86_64_gnullvm"
-version = "0.52.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "24d5b23dc417412679681396f2b49f3de8c1473deb516bd34410872eff51ed0d"
-
 [[package]]
 name = "windows_x86_64_gnullvm"
 version = "0.53.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0ffa179e2d07eee8ad8f57493436566c7cc30ac536a3379fdf008f47f6bb7ae1"
 
-[[package]]
-name = "windows_x86_64_msvc"
-version = "0.52.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "589f6da84c646204747d1270a2a5661ea66ed1cced2631d546fdfb155959f9ec"
-
 [[package]]
 name = "windows_x86_64_msvc"
 version = "0.53.1"
@@ -1494,35 +1094,6 @@ version = "0.51.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d7249219f66ced02969388cf2bb044a09756a083d0fab1e566056b04d9fbcaa5"
 
-[[package]]
-name = "writeable"
-version = "0.6.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9edde0db4769d2dc68579893f2306b26c6ecfbe0ef499b013d731b7b9247e0b9"
-
-[[package]]
-name = "yoke"
-version = "0.8.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "72d6e5c6afb84d73944e5cedb052c4680d5657337201555f9f2a16b7406d4954"
-dependencies = [
- "stable_deref_trait",
- "yoke-derive",
- "zerofrom",
-]
-
-[[package]]
-name = "yoke-derive"
-version = "0.8.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b659052874eb698efe5b9e8cf382204678a0086ebf46982b79d6ca3182927e5d"
-dependencies = [
- "proc-macro2",
- "quote",
- "syn",
- "synstructure",
-]
-
 [[package]]
 name = "zerocopy"
 version = "0.8.33"
@@ -1542,57 +1113,3 @@ dependencies = [
  "quote",
  "syn",
 ]
-
-[[package]]
-name = "zerofrom"
-version = "0.1.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "50cc42e0333e05660c3587f3bf9d0478688e15d870fab3346451ce7f8c9fbea5"
-dependencies = [
- "zerofrom-derive",
-]
-
-[[package]]
-name = "zerofrom-derive"
-version = "0.1.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d71e5d6e06ab090c67b5e44993ec16b72dcbaabc526db883a360057678b48502"
-dependencies = [
- "proc-macro2",
- "quote",
- "syn",
- "synstructure",
-]
-
-[[package]]
-name = "zerotrie"
-version = "0.2.3"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2a59c17a5562d507e4b54960e8569ebee33bee890c70aa3fe7b97e85a9fd7851"
-dependencies = [
- "displaydoc",
- "yoke",
- "zerofrom",
-]
-
-[[package]]
-name = "zerovec"
-version = "0.11.5"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6c28719294829477f525be0186d13efa9a3c602f7ec202ca9e353d310fb9a002"
-dependencies = [
- "yoke",
- "zerofrom",
- "zerovec-derive",
-]
-
-[[package]]
-name = "zerovec-derive"
-version = "0.11.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "eadce39539ca5cb3985590102671f2567e659fca9666581ad3411d59207951f3"
-dependencies = [
- "proc-macro2",
- "quote",
- "syn",
-]
diff --git a/filtermail/Cargo.toml b/filtermail/Cargo.toml
index af268a99..afa5bc4c 100644
--- a/filtermail/Cargo.toml
+++ b/filtermail/Cargo.toml
@@ -16,10 +16,6 @@ serini = "0.2.2"
 tokio = { version = "1.49.0", features = ["full"] }
 thiserror = "2.0.18"
 mailparse = "0.16.1"
-lettre = { version = "0.11.19", default-features = false, features = [
-  "smtp-transport",
-  "tokio1",
-] }
 log = "0.4.29"
 env_logger = "0.11.8"
 governor = "0.10.4"
diff --git a/filtermail/src/error.rs b/filtermail/src/error.rs
index 4f6c9c80..76dd251d 100644
--- a/filtermail/src/error.rs
+++ b/filtermail/src/error.rs
@@ -10,4 +10,23 @@ pub enum Error {
     Io(#[from] std::io::Error),
     #[error("OpenPGP packet header is truncated - can't validate!")]
     TruncatedHeader,
+    #[error("Unable to send email, Error during {context}, server said: {raw_smtp_answer}")]
+    MailSend {
+        context: String,
+        raw_smtp_answer: String,
+    },
+}
+
+impl Error {
+    /// Formats [`Error`] as an SMTP response.
+    pub fn smtp_response(&self) -> String {
+        match self {
+            // We transparently pass postfix/milter errors reported on reinjection
+            Error::MailSend {
+                raw_smtp_answer, ..
+            } => raw_smtp_answer.clone(),
+            Error::TruncatedHeader => self.to_string(),
+            _ => "451 Local error".to_string(),
+        }
+    }
 }
diff --git a/filtermail/src/inbound.rs b/filtermail/src/inbound.rs
index fb554dde..7d295b38 100644
--- a/filtermail/src/inbound.rs
+++ b/filtermail/src/inbound.rs
@@ -5,11 +5,10 @@ use crate::config::Config;
 use crate::message::{check_encrypted, is_securejoin};
 use crate::smtp_server::SmtpHandler;
 use async_trait::async_trait;
-use lettre::{AsyncSmtpTransport, AsyncTransport, Tokio1Executor};
 use mailparse::{MailHeaderMap, parse_mail};
 
 pub use crate::smtp_server::Envelope;
-use crate::utils::{extract_address, format_smtp_error};
+use crate::utils::extract_address;
 
 /// Handler for incoming SMTP messages.
 pub struct IncomingBeforeQueueHandler {
@@ -80,30 +79,12 @@ impl SmtpHandler for IncomingBeforeQueueHandler {
     async fn reinject_mail(&self, envelope: &Envelope) -> Result<(), String> {
         log::debug!("Re-injecting the mail that passed checks");
 
-        let mailer = AsyncSmtpTransport::<Tokio1Executor>::builder_dangerous("localhost")
-            .port(self.config.postfix_reinject_port_incoming)
-            .build();
-
-        let envelope_data = lettre::address::Envelope::new(
-            Some(
-                envelope
-                    .mail_from
-                    .parse()
-                    .map_err(|e| format!("Invalid from address: {}", e))?,
-            ),
-            envelope
-                .rcpt_to
-                .iter()
-                .map(|addr| addr.parse())
-                .collect::<Result<Vec<_>, _>>()
-                .map_err(|e| format!("Invalid to address: {}", e))?,
-        )
-        .map_err(|e| format!("Failed to create envelope: {}", e))?;
-
-        mailer
-            .send_raw(&envelope_data, &envelope.data)
+        crate::smtp_client::send(self.config.postfix_reinject_port_incoming, envelope)
             .await
-            .map_err(format_smtp_error)?;
+            .map_err(|e| {
+                log::warn!("Failed to re-inject mail: {}", e);
+                e.smtp_response()
+            })?;
 
         Ok(())
     }
diff --git a/filtermail/src/main.rs b/filtermail/src/main.rs
index 12b2048d..62db8c79 100644
--- a/filtermail/src/main.rs
+++ b/filtermail/src/main.rs
@@ -30,6 +30,7 @@ pub(crate) mod inbound;
 pub(crate) mod message;
 pub(crate) mod openpgp;
 pub(crate) mod outbound;
+pub(crate) mod smtp_client;
 pub(crate) mod smtp_server;
 pub(crate) mod utils;
 
diff --git a/filtermail/src/outbound.rs b/filtermail/src/outbound.rs
index da7381be..22261aae 100644
--- a/filtermail/src/outbound.rs
+++ b/filtermail/src/outbound.rs
@@ -5,10 +5,9 @@ use crate::config::Config;
 use crate::message::{check_encrypted, is_securejoin, recipient_matches_passthrough};
 pub use crate::smtp_server::Envelope;
 use crate::smtp_server::SmtpHandler;
-use crate::utils::{extract_address, format_smtp_error};
+use crate::utils::extract_address;
 use async_trait::async_trait;
 use governor::{DefaultKeyedRateLimiter, Quota, RateLimiter};
-use lettre::{AsyncSmtpTransport, AsyncTransport, Tokio1Executor};
 use mailparse::{MailHeaderMap, parse_mail};
 
 /// Handler for outgoing SMTP messages.
@@ -127,30 +126,12 @@ impl SmtpHandler for OutgoingBeforeQueueHandler {
     async fn reinject_mail(&self, envelope: &Envelope) -> Result<(), String> {
         log::debug!("Re-injecting the mail that passed checks");
 
-        let mailer = AsyncSmtpTransport::<Tokio1Executor>::builder_dangerous("localhost")
-            .port(self.config.postfix_reinject_port)
-            .build();
-
-        let envelope_data = lettre::address::Envelope::new(
-            Some(
-                envelope
-                    .mail_from
-                    .parse()
-                    .map_err(|e| format!("Invalid from address: {}", e))?,
-            ),
-            envelope
-                .rcpt_to
-                .iter()
-                .map(|addr| addr.parse())
-                .collect::<Result<Vec<_>, _>>()
-                .map_err(|e| format!("Invalid to address: {}", e))?,
-        )
-        .map_err(|e| format!("Failed to create envelope: {}", e))?;
-
-        mailer
-            .send_raw(&envelope_data, &envelope.data)
+        crate::smtp_client::send(self.config.postfix_reinject_port, envelope)
             .await
-            .map_err(format_smtp_error)?;
+            .map_err(|e| {
+                log::warn!("Failed to re-inject mail: {}", e);
+                e.smtp_response()
+            })?;
 
         Ok(())
     }
diff --git a/filtermail/src/smtp_client.rs b/filtermail/src/smtp_client.rs
new file mode 100644
index 00000000..a2dee266
--- /dev/null
+++ b/filtermail/src/smtp_client.rs
@@ -0,0 +1,78 @@
+use crate::smtp_server::Envelope;
+use std::net::{IpAddr, SocketAddr};
+use tokio::io::{AsyncBufReadExt, AsyncWriteExt, BufStream};
+use tokio::net::TcpSocket;
+
+const LOCALHOST: IpAddr = IpAddr::V4(std::net::Ipv4Addr::new(127, 0, 0, 1));
+
+/// Sends an email using an SMTP server at `localhost:<smtp_port>`.
+pub async fn send(smtp_port: u16, envelope: &Envelope) -> Result<(), crate::error::Error> {
+    let socket = TcpSocket::new_v4()?;
+
+    // Disable Nagle's algorithm.
+    socket.set_nodelay(true)?;
+
+    let stream = socket
+        .connect(SocketAddr::new(LOCALHOST, smtp_port))
+        .await?;
+
+    let mut buf_stream = BufStream::new(stream);
+    let mut response = String::new();
+
+    macro_rules! smtp_write {
+        ($command: expr) => {
+            buf_stream.write_all($command).await?;
+            buf_stream.flush().await?;
+        };
+    }
+
+    macro_rules! smtp_read {
+        ($context:expr, $expected_code:expr) => {
+            buf_stream.read_line(&mut response).await?;
+            if !response.starts_with($expected_code) {
+                return Err(crate::error::Error::MailSend {
+                    context: $context.to_string(),
+                    raw_smtp_answer: response.clone(),
+                });
+            }
+            response.clear();
+        };
+    }
+
+    macro_rules! smtp_cmd {
+        ($command:expr, $context:expr, $expected_code:expr) => {
+            smtp_write!($command);
+            smtp_read!($context, $expected_code);
+        };
+    }
+
+    // Read initial greeting
+    smtp_read!("initial greeting", "220");
+
+    // Greet (Using HELO as we don't want to deal with extended SMTP anyway.)
+    smtp_cmd!(b"HELO localhost\r\n", "HELO", "250");
+
+    // MAIL FROM
+    smtp_cmd!(
+        format!("MAIL FROM:<{}>\r\n", envelope.mail_from).as_bytes(),
+        "MAIL FROM",
+        "250"
+    );
+
+    // RCPT TO
+    for rcpt in &envelope.rcpt_to {
+        smtp_cmd!(
+            format!("RCPT TO:<{}>\r\n", rcpt).as_bytes(),
+            "RCPT TO",
+            "250"
+        );
+    }
+
+    // DATA
+    smtp_cmd!(b"DATA\r\n", "DATA", "354");
+    smtp_write!(&envelope.data);
+    smtp_write!(b".\r\n");
+    smtp_read!("end of DATA", "250");
+
+    Ok(())
+}
diff --git a/filtermail/src/utils.rs b/filtermail/src/utils.rs
index 356270c7..563cad1e 100644
--- a/filtermail/src/utils.rs
+++ b/filtermail/src/utils.rs
@@ -1,5 +1,4 @@
 use mailparse::MailAddr;
-use std::error::Error;
 
 /// Extracts the first email address found in SMTP command or email header.
 ///
@@ -27,29 +26,6 @@ pub fn extract_address(input: &str) -> Option<String> {
         })
 }
 
-/// Formats SMTP error to be able to send it back to postfix.
-pub fn format_smtp_error(error: lettre::transport::smtp::Error) -> String {
-    if let Some(code) = error.status() {
-        format!(
-            "{} {}",
-            code,
-            error
-                .source()
-                .map(ToString::to_string)
-                .unwrap_or("Unknown error".to_string())
-        )
-    } else {
-        // Default to 451, most probably means some internal service error (e.g. milter)
-        format!(
-            "451 {}",
-            error
-                .source()
-                .map(ToString::to_string)
-                .unwrap_or("Unknown error".to_string())
-        )
-    }
-}
-
 #[cfg(test)]
 mod tests {
     use super::*;