refactor: Use a custom, minimal SMTP client instead of lettre (#33)

This disables the Nagle's algorithm on re-insertion.

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

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;