feat: Save rejected messages to /tmp (#55)

filtermail/src/inbound.rs

@@ -6,7 +6,7 @@ use crate::dkim_verifier::DkimVerifier;
 use crate::message::{check_encrypted, is_securejoin};
 pub use crate::smtp_server::Envelope;
 use crate::smtp_server::SmtpHandler;
-use crate::utils::{AddressDomain, extract_address};
+use crate::utils::{AddressDomain, extract_address, log_eml};
 use async_trait::async_trait;
 use mailparse::{MailHeaderMap, parse_mail};
 use std::str::FromStr;
@@ -69,8 +69,18 @@ impl SmtpHandler for IncomingBeforeQueueHandler {
                 }
             }
             AddressDomain::Name(domain) => {
-                if !self.skip_dkim {
-                    self.dkim_verifier.verify(&envelope.data, &domain).await?;
+                if !self.skip_dkim
+                    && let Err(e) = self.dkim_verifier.verify(&envelope.data, &domain).await
+                {
+                    let eml_path = log_eml("dkim-verify", &envelope.data)
+                        .await
+                        .map(|path| path.to_string_lossy().to_string())
+                        .unwrap_or_else(|e| {
+                            log::error!("Failed to save rejected message to file: {e}");
+                            "ERR".to_string()
+                        });
+                    log::info!("Rejected message stored at: {eml_path}");
+                    return Err(e);
                 }
             }
         }

filtermail/src/utils.rs

@@ -1,4 +1,5 @@
 use mailparse::MailAddr;
+use std::path::PathBuf;
 use std::str::FromStr;
 
 /// Extracts the first email address found in SMTP command or email header.
@@ -75,6 +76,23 @@ impl FromStr for AddressDomain {
     }
 }
 
+/// Logs email to `/tmp/filtermail-rejected/<reason>/<timestamp>.eml`
+/// and returns the file path.
+///
+/// Returns [`crate::error::Error`] on IO error.
+pub async fn log_eml(reason: &str, data: &[u8]) -> Result<PathBuf, crate::error::Error> {
+    let timestamp = std::time::SystemTime::now()
+        .duration_since(std::time::UNIX_EPOCH)
+        .unwrap_or_default()
+        .as_secs();
+    let filename = format!("{timestamp}.eml");
+    let mut path = PathBuf::from(format!("/tmp/filtermail-rejected/{reason}"));
+    tokio::fs::create_dir_all(&path).await?;
+    path.push(filename);
+    tokio::fs::write(&path, data).await?;
+    Ok(path)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;