The fundamental SMTP system engineering point here is that the MX
must know how to accept only valid recipients, and reject invalid recipients.
If Imail is setup correctly, it will bounce invalid recipients from
the MX gateway, which then generates an NDR msg to the envelope sender.
Running a "recipient ignorant" MX is a horrible mistake. A lot of
the envelope senders addressing invalid recipients are forged, so the
MX not only gets flooded with generating NDR messges, those NDR
messages aren't deliverable, a total waste of MX resources.
The AV function and content-scanning function need to be AFTER the MX does:
1) bouncing of invalid recips and
2) envelope anti-abuse filtering .
This setup feeds minimized traffic to the "expensive" AV and
content-scanning functions.
So when setting up an MX gateway, always make sure it has its own
database of legit recips.
Len
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