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I've set up a website for someone and redirected his domain to point to my server. He has his own exchange mail server he still wants to use. He couldnt recieve mails after the domain transfer.

I've added a MX record with the servername of his exchange server, and now he can send and recieve. Its under 24 hours since his mail stopped recieving.

Now my question: Is there a way to get the mails he should have recieved before I added the MX record?

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...and is adding the MX record the only thing I need to do? :) – Greenie Dec 8 '10 at 10:34
It's all dependent on the sending MTA. Some will retry at regular intervals for up to 48 hours but it's specific to the configuration of each MTA. – joeqwerty Dec 8 '10 at 13:26
You'll also need an A record for the Exchange server. The MX record "points" to the A record. – joeqwerty Dec 8 '10 at 13:35

Email servers will retry sending the failed emails, but if the downtime period is "long" you may lose your emails as they will not retry forever. I am not sure how long this period is.

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You might want to check RFC2821 and the corresponding implementation of a potential mail server being setup on your server.

If no MX records are found, but an A RR is found, the A RR is treated as if it was associated with an implicit MX RR, with a preference of 0, pointing to that host. Section 5 of RFC 2821

So the question should be whether your server might have accepted/rejected the emails. In most cases your Server would have sent something like SMTP 550 error, 5.1.1 User Unknown because it wasn't aware it should accept mails for these recipients.

So at least no mails should have been lost without being handled since the recipients should have been informed by your mailserver.

Otherwise if you don't run any mail server on the A RRs address of the Domain the mail servers should try to resent the emails.

This only applies if all mailservers involved behave RFC conform

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