IANA
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... service extensions it supports. Extensions to the SMTP service
are registered with the IANA. This framework does not require
modification of existing SMTP ...
... EHLO keyword value. Each
service extension registered with the IANA must be defined in an RFC.
Such RFCs must either be on the standards-track or must define an
...
... approved experimental SMTP service extension registered with IANA. A
conforming server must not offer non "X" prefixed keyword values that
are not described in a registered extension.
...
... Initial IANA Registry ...
...
All esmtp-keyword values must be registered as part of the IANA
registration process described above. This definition only provides
the framework for future extension; no extended MAIL FROM ...