Mail processing engine:
1. It trusts 'From' header, probably the better way should be used (IMAP envelope data?)
2. If SMTP fails, message will be lost
3. It checks only 'To:' header, but users might 'Cc:' to mail list
4. Check up for failures in pending pool (extremely rare case, but anyway)
5. Allocation checks
6. Missing logging of different events