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 pipe-lining. * Support for Internationalized Domain Names (IDN). * DSN (Delivery Status Notification) support. * RMQS (Remote Message Queue Starting) support (ETRN keyword). * IPv6 support.