Extended SMTP/External Links
- Please sort and annotate in a user-friendly manner and consider archiving the URLs behind the links you provide. See also related web sources.
SMTP Extension Model - "The Extension Model", section 2.2 of RFC-5321, "Simple Mail Transfer Protocol"
IANA Registries - Official registries for special domain names, reserved numbers, and protocol parameters.
Simple Mail Transfer Protocol (SMTP) Service Extensions