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I'm migrating a Mailman installation from Server A to Server B. Server B is fully configured and ready for the move, but I would like to test it before changing my domain's MX entries. If I were testing HTTP, I would just modify my /etc/hosts file and test it that way, but I'm not familiar with a similar technique for email. Is there a way to test a mail server migration before I actually update the DNS entries, or do I just have to make the change and hope for the best when the name server changes propagate?

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First, a mandatory joeqwerty quote: "DNS RECORDS ARE NOT PROPOGATED, THEY ARE CACHED."

You could test the new MX record by creating a zone in your the DNS server used by another email server matching the domain's name and putting the new MX record in that zone, effectively "trumping" any Internet DNS server for the destination domain. Then you could send email through this email server such that it will use the MX record specified in the zone hosted by its DNS server. Of course, this implies that you have control over the DNS server used by this hypothetical other email server. That's about as realistic a test I can think of. Convolulted, but realistic.

Make sure you've run SMTP conversations against the new email server using TELNET and verified that the server is delivering email properly. Also make sure that the server, if it's behind a NAT firewall, has had TCP port 25 properly forwarded from its public IP to its private IP (i.e. test it with TELNET from somewhere outside your LAN).

If you've done all that you're probably good to go.

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