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I was considering setting up my own mail server and one of the things I've read about is that I must setup reverse DNS to do not get caught by a spam filter.

Is that really a must? Aren't SPF and DKIM enough to prevent be labeled as spam?

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Your question,

"Is having a PTR record a must for sending mail servers?"

is not that easy to answer fully.

Is it an requirement to be able to send mail?

No, you can send mail from any address.

"Can SPF and DKIM can replace a PTR record?"

the answer is no, each of these have a different function.

and when we combine the question and look at the full problem, we have to consider that as mail senders we have to invoke trust into ourselves as a trusted party that plays nice, does not send spam etc. (e.a. we are a good sender)

In order to invoke that trust we should implement a PTR record for our mail server, a SPF record and DKIM record to protect our messages and canonical source of messages.

In conclusion, is a PTR really needed when you host your own sending mail server? Yes, because we want others to trust us as a proper mail sender.

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Standards don't require PTR to send mail, but the real servers that receive mails quite frequently require valid PTRs. Why? To partially eliminate botnets' spam. PTRs are quite hard to obtain with some inexpensive network providers and by conjecture, many botnet members wouldn't have a valid PTR.

All the other DNS records are much easier to set up for yourself than PTR (I won't get into quite obscure technical reasons).

SPF and DKIM have nothing to do with spam. They are anti-spoofing. In fact, many spammers have perfect DMARC, DKIM, SPF records. You can be assured the spam they send is not spoofed, hurray.

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