Listens and forwards SMTP messages to another sever.
It's mainly used as a bridge between old mail clients and mail servers that don't accept old TLS versions anymore (like AWS SES).
Edit the settings in smtpfw.json.
WARNING: This cannot be complied for Windows XP or older because dependencies don't support Go v1.10 (the last version supporting Windows XP) anymore. See the Python 3.4 variant instead.