Re: Is this a potential problem?

I have a server with IIS SMTP running. I am suspecting that some
emails are not being delivered since I have some events (ID 4000) in
my Event log "Message delivery to the remote domain 'zmail.com'
failed for the following reason: The remote server did not respond
to a connection attempt."

Your suspicion is correct. However, the Event Log should be only a
pitstop on your way to the real IIS logs. The EL gives a sort of
summary of a bad session; the real, SMTP-level failure will be noted
in the logs.

3. The server is configured with multiple IPs. The SMTP is set up to
receive on one of these mail.domain.com (X.X.X.2). The sending IP is
however not the same server.domain.com (X.X.X.1)

That can indeed cause delivery problems. It means you fail a full
PTR-A-EHLO-IP roundtrip set of anti-spam tests. Not at all servers are
so rigorous with these tests, which explains the interleaved failures
and successes (though without your logs, it's impossible to say this
is firmly _the_ reason).

(I don't think you can force IIS SMTP to send from a specific IP).

The source IP will be the primary IP of a NIC, yes.

The bottom line is that you need to worry about outbound delivery
first and foremost, not inbound connections to your MX. So if a
virtual server is going to make deliveries from a given source IP, it
needs to have its HELO/EHLO (FQDN in IIS) match *that* IP's PTR, not
the PTR of an IP the receiving public will never see.

That will naturally change the hostname shown in the response to
inbound connections as well, but that's in practice a non-issue.

I have added an entry with a high priority number ala: (same as
parent folder) Mail Exchanger [100] mail.domain.com Just in case a
remote server would be checking if a MX record was in place for this
server.

There is no reason whatsoever to do this. HELO/EHLO hostnames of
outbound connections do not need to have MX records. (Note this
applies whether or not the same virtual server also accepts inbound
mail for a set of domains. You need MX records for the _domains_ that
receive mail.) All you've done is cluttered your DNS zone with
non-responsive servers.

--Sandy
.