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In our company we host our own exchange mail server, and this sends mails from our internal IP. The DNS PTR record is configured for this and everything gets delivered.

We also have some web applications externally hosted that send emails. These keep getting stuck in spam filters and I've been told a DNS PTR record needs adding.

Both services send from the same domain name, but from 2 very different IPs.

From what I have read here (Is it bad to have the reverse DNS for two IPs point to the same domain name?) and here (Why multiple PTR records in DNS is not recommended?) it sounds like simply adding a second record with the web IP may cause issues. At the same time this seams like a fairly standard set up so if anyone can let me know the best way to set this up it would be great.

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If there's no PTR record at all for your web application hosts that can be enough for mail to get rejected or stuck in spam filters. So to begin with having any sort of PTR record is a good start.

There's no requirement that the IP has to resolve to the mail domain. Generally whatever the host announces itself as to remote mail servers (with the EHLO verb) needs to be resolvable to an IP that should match the IP used by the host itself, and that IP needs to be resolvable back to a hostname that ideally matches the original.

You should also set up SPF records which can state via DNS that both your Exchange server and your web application hosts are allowed to send on behalf of your mail domain. This will help with mail getting delivered.

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