feat(transport): Remote delivery over SMTP (#104)

Implements a new mode "transport",
that allows filtermail to be used for
remote delivery.

In transport mode, filtermail listens for LMTP
connections, splits messages by domain
and performs delivery to remote MTAs over SMTP.

SMTP client tries to open socket on all resolved
addresses in parallel and uses one that succeeds
the first, fixing the issue described in:
chatmail/relay#900

Extends the built-in SMTP client
with STARTTLS support.

Extends the built-in SMTP server
with LMTP greeting support.

Groundwork required for mxdeliv endpoint
(HTTP channel for MTA-to-MTA communication):
chatmail/relay#900

Signed-off-by: Jagoda Ślązak <jslazak@jslazak.com>

filtermail/src/utils.rs

@@ -1,3 +1,5 @@
+use hickory_resolver::TokioResolver;
+use hickory_resolver::name_server::TokioConnectionProvider;
 use mailparse::MailAddr;
 use std::path::PathBuf;
 use std::str::FromStr;
@@ -30,7 +32,7 @@ pub fn extract_address(input: &str) -> Option<String> {
 
 /// Domain part of an email address, either a domain-literal (IP address in square brackets with
 /// optional protocol prefix) or a regular domain name.
-#[derive(Debug, PartialEq, Eq)]
+#[derive(Debug, PartialEq, Eq, Hash, Clone, PartialOrd, Ord)]
 pub enum AddressDomain {
     /// Domain literal, e.g.
     /// - `192.0.2.0` in `test@[192.0.2.0]`,
@@ -76,6 +78,15 @@ impl FromStr for AddressDomain {
     }
 }
 
+impl AsRef<str> for AddressDomain {
+    fn as_ref(&self) -> &str {
+        match self {
+            AddressDomain::Literal(literal) => literal.as_ref(),
+            AddressDomain::Name(name) => name.as_ref(),
+        }
+    }
+}
+
 /// Logs email to `/tmp/filtermail-rejected/<reason>/<timestamp>.eml`
 /// and returns the file path.
 ///
@@ -93,6 +104,23 @@ pub async fn log_eml(reason: &str, data: &[u8]) -> Result<PathBuf, crate::error:
     Ok(path)
 }
 
+/// Creates a DNS resolver with DNSSEC enabled and system configuration (resolv.conf).
+pub fn build_resolver() -> Result<TokioResolver, crate::error::Error> {
+    let dns_resolver = {
+        let mut builder = TokioResolver::builder(TokioConnectionProvider::default())?;
+        // https://github.com/hickory-dns/hickory-dns/issues/3519
+        builder.options_mut().validate = true;
+        builder.build()
+    };
+
+    assert!(
+        dns_resolver.options().validate,
+        "incorrect resolver config: DNSSEC disabled; exiting"
+    );
+
+    Ok(dns_resolver)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;