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Let me start with the fact that things are quite weird in our organization.

Now, we have to place a mailserver for a DNS zone that is under our control, for example xxx.yyy.zzz. However, we can't place it physically in the subnet allocated for this zone.

At the current moment we have two possibilities. Place it in a different subnet with it's own DNS zone and insert it into the rDNS for that zone. That is, our DNS points at that IP, their rDNS points at our domain name.

Alternatively, we can place it in a different subnet and rDNS lookup won't work.

My question here is, is either of these setting "mail-friendly". That is, will mail servers, mail clients, spam detection, etc. tolerate such a setting or there is a great chance e-mails from such a server will be dropped, marked as spam and so on.

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You really should have an rdns lookup, and if the rDNS and the DNS don't match, some of the antispam products out there will take that as an increased likelihood that the server is spamming. But it's better to have a DNS/rDNS mismatch than to have no rDNS at all.

You do not have to have the DNS and rDNS handled by the same servers, though - it's not that uncommon for rDNS to be handled by an ISP, while DNS is handled by the company server.

Further, I'd advise you to make sure that the hostname used by your server in HELO also resolves, preferably to the same thing as the actual server.

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