1 Will Declude still scan the messages?
yes, but since the postfix box (I assume IMGate) will reject 70+% of the incoming, declude and friends will have tons less work to do. That will your biggest improvement.
2 If so, when a message is Virus Positive, will Imail still forward the email to the gateway pf server?
it better not
3 In your opinion, is this a good alternative?
yes
The idea is to have the PostFix server as a SMTP Gateway only for Imail, our users will have to send the mails to imail, and imail to the postftix.
You can run auth relay through Imail, or you can use postfix do to pop-before-smtp relaying, off-loading the outbound mail from Imail (but you lose the AV scanning of Declude). postfix can be configured to block dangerous attachments, so your outbound will still have significant infection blocking.
If your Imail box is still sluggish after adding the SMTP gateway box (which would be a surprising first occurrence), the next step would be to use another box dedicated to AV scanning. An big IMGate user in NJ does the 3-level solution, for several 100K msgs/day.
As i see it, Imail still manages the Relay Rules, SMTP Auth, Etc,
yes
cause the PostFix server will only relay from Imails Server IP, so Internet delivery process wont consume resources and time at the imail server.
but your biggest traffic is not outbound, but inbound, by probably 8 to 1, so the biggest assist you get from IMGate would be rejecting 70% of the inbound traffic and keeping it off the Imail machine.
Secondarily, Imail, if its version 7 queue doesn't get screwed up, will be drop its outbound on postfix at high speed with no DNS queries, meaning a lot of long-lived Imail SMTP processes will not be needed, liberating CPU and memory and disk i/o (no deferrals in Imail queue).
You should run BIND on the postfix box.
Is this scheme good?
It's worked very well for everybody. I can think of references in 3 Spanish-speaking countries (Puerto Rico, Bolivia, and Florida) that had exactly your problem and have recovered excellent Imail performance by doing exactly what you're talking about.
Len
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