feat: Initial implementation

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

filtermail/src/utils.rs

@@ -0,0 +1,74 @@
+use mailparse::MailAddr;
+use std::error::Error;
+
+/// Extracts the first email address found in SMTP command or email header.
+///
+/// Return `None` if parsing fails.
+///
+/// Returns the first address if multiple are present.
+pub fn extract_address(input: &str) -> Option<String> {
+    // TODO: at this point it's probably simpler to use regex ;p
+    let input_lower = input.to_lowercase();
+    let mut trimmed = input_lower
+        .trim_start_matches("mail from:")
+        .trim_start_matches("rcpt to:");
+    trimmed = trimmed
+        .split_once("=")
+        .map(|(address_raw, _)| {
+            address_raw
+                .rsplit_once(' ')
+                .map(|(addr, _)| addr)
+                .unwrap_or(address_raw)
+                .trim()
+        })
+        .unwrap_or(trimmed);
+
+    mailparse::addrparse(trimmed)
+        .ok()
+        .and_then(|addr| match addr.first() {
+            Some(MailAddr::Single(single)) => Some(single.addr.clone()),
+            Some(MailAddr::Group(group)) => group.addrs.first().map(|single| single.addr.clone()),
+            None => None,
+        })
+}
+
+/// 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::*;
+    use rstest::*;
+
+    #[rstest]
+    #[case("MAIL FROM:<t1@example.org>", Some("t1@example.org".to_string()))]
+    #[case("MAIL FROM:<t2@example.org> SOMETHING=SOMETHING OTHER=OTHER", Some("t2@example.org".to_string()))]
+    #[case("RCPT TO:<t3@example.org>", Some("t3@example.org".to_string()))]
+    #[case("mail from:<t4@example.org>", Some("t4@example.org".to_string()))]
+    #[case("Foo Bar <t5@example.org>", Some("t5@example.org".to_string()))]
+    #[case("t6@example.org", Some("t6@example.org".to_string()))]
+    fn test_extract_address(#[case] input: &str, #[case] expected: Option<String>) {
+        let result = extract_address(input);
+        assert_eq!(result, expected)
+    }
+}