It sounds like you are trying to invent something here. :)
It sound like you are trying to confuse something here :))
There are 3 elements available in the SMTP sessions to block on:
1. SMTP client (that thingy TCP-connecting to your SMTP server). the block is against the IP or PTR hostname of the SMTP client, including full subnets and sub-domains.
2. HELO hostname. the block is against the fqdn. But also, it must not contain invalid (DNS) characters, must be a fqdn, and must be in DNS A or MX records. This is forgeable by the SMTP server software, so anti-forgery is very effective.
3. MAIL FROM:<envelope.sender> is against:
sender@ [EMAIL PROTECTED] sender.domain
Again, envelope sender is easily forgeable, widely forged, so anti-forgery is very effective. RHS, right hand side, is a useless term that refers to the right hand side of the @ sign of the envelope sender, aka more precisely as the ESD, envelope sender domain. guess what ERD is?
Filtering on the above is called "envelope rejection" somewhat loosely (STMP client and HELO hostname aren't in the envelope), but rejecting after, no later than:
RCPT TO:<envelope.recipient>
... is rejecting after the envelope is completely known. This is the most efficient point to reject, since only a few bytes and seconds of SMTP dialog are the cost, vs. accepting many KB or MB of DATA arriving over 10's of seconds or minutes.
All other filtering policies require receipt of the DATA command for content-scanning, which includes 1) SMTP header scanning, and 2) msg body scanning. Even if you don't "scan" the DATA for string matches, just receiving the DATA of spam messages is costly.
It's all really very simple and making up terms as you go along is very unhelpful.
Len
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