Implements the following SMTP-related RFCs:
- RFC 1870/STD 10: Message Size Declaration - RFC 2034: Enhanced-Status-Codes - RFC 2195: CRAM-MD5 (SASL Authentication) - RFC 2595/4616: TLS & PLAIN (SASL Authentication) - RFC 2831: DIGEST-MD5 authentication mechanism (obsoleted by RFC 6331) - RFC 2920/STD 60: Pipelining - RFC 3207: Secure SMTP over TLS - RFC 3463: Enhanced Mail System Status Codes - RFC 4422: SASL Authentication (for DIGEST-MD5) - RFC 4954: Authentication - RFC 5321: Simple Mail Transfer Protocol - RFC 6152/STD 71: 8bit-MIMEtransport - RFC 6409/STD 72: Message Submission for Mail - RFC 6531: Internationalized Email - XOAUTH2: https://developers.google.com/gmail/xoauth2_protocolTODO:
- RFC 1845: CHECKPOINT - RFC 2852: DELIVERYBY - RFC 3030: BINARYMIME/CHUNKING - RFC 3461: DSN - RFC 3865: NO-SOLICITING - RFC 3885: MTRK - RFC 4141: CONPERM/CONNEG - RFC 4405: SUBMITTER - RFC 4468: BURL - RFC 4865: FUTURERELEASE - RFC 6710: MT-PRIORITY - RFC 7293: RRVS