It forwards messages to an SMTP server which does the delivery.
Features include:
* Sendmail compatible interface (command line options and exit codes).
* Authentication methods PLAIN,LOGIN,CRAM-MD5,DIGEST-MD5,GSSAPI,and NTLM
* TLS/SSL both in SMTP-over-SSL mode and in STARTTLS mode.
* Fast SMTP implementation using command pipelining.
* Support for Internationalized Domain Names (IDN).
* DSN (Delivery Status Notification) support.
* RMQS (Remote Message Queue Starting) support (ETRN keyword).
* IPv6 support.
