NOTES on rfc 2476.
Message Submission RFC:  http://www.faqs.org/rfcs/rfc2476.html


Differentiates between MTA's that receive relays, and MSA's Message
Submission Agent, which receives from MUAs.

The idea is that strict validation be done on MTA traffic, and 
authentication and message-completion be performed on MSA traffic.

Prefer that "smtp" port receive only MTA requests and "submission" port
receives only MUA submissions.

MAIL FROM of "<>" definitely permissible.

For a reject, it's up to server whether to refuse send or to accept
(or refuse) and then send a bounce).  Of course, must be able to 
generate a reply addr to send a bounce.  Immediate refusal is preferred.

Some enhanced status codes for a MSA are listed.

Expand only un-qualified domains; reject partially-qualified domains.


MODIFY SUBMITTED MESSAGES IN FOLLOWING WAYS:

    Add "Sender:" if valid sender email addr known and is not given in
    "From:".

    Fix or add "Date", "Message-ID"