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Could someone help answer a few questions regarding the networking / DNS requirements for an SMTP server? Assume the server is setup already.

Is a MX record required for SMTP? TCP port 25 open to the server? Any other DNS records?

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You may want to consider an SPF record as well.

http://en.wikipedia.org/wiki/Sender_Policy_Framework

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You need port 25 open. An MX record is preferred, but any RFC-compliant server will fall back to the A record for your domain if one doesn't exist.

  • Thanks Bill. So even though the server is not receiving mail, just sending, a MX is preferred? And thanks for confirming, TCP 25. It <b>does</b> seem to be working without, however. – Bill Smith Jul 6 '10 at 18:21
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    The phrase "server" implies you're receiving. If you're just sending, you don't need any of that. – Bill Weiss Jul 6 '10 at 18:30
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I noticed you used the relay tag - that functionality may be disabled by default, depending on what you're using. You'd need to add the upstream host to the relay host's list of trusted SMTP sources.

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Outbound mail servers don't need an MX record but be advised that some email providers (most notably AOL) will check for a reverse DNS (PTR) record for the FQDN your server provides when communicating. I'd suggest adding a PTR.

-M

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