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We are in the process of migrating from our old domain to a new one, part of which involves migrating mailboxes from Exchange 2003 to Exchange 2007. A bunch of users have been migrated already without problems. However, one of the users is having trouble receiving emails from others. When someone sends to him, they get an Undeliverable NDR that says "A configuration error in the e-mail system caused the message to bounce between two servers or to be forwarded between two recipients." The message shows the user's distinguished name as /OU=OurDomain/CN=Recipients/CN=USER57137172. The user's account name should just be "USER", so I don't know where the extra numbers ("57137172") are coming from. Thanks in advance.

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Does the affected user have "strange" characters in their Exchange alias (like any of "$^#\;/= -,")? That's typically what causes a random number to be appended to a legacyExchangeDN. (See for reference.)

Was this a simple "Move Mailbox" between two Exchange Server computers that are members of the same AD forest or is this a cross-forest mailbox move (or something screwy like a "export to PST and import from PST 'mailbox move")?

The mail flow problem sounds like people are trying to send messages to the use at the old legacyExchangeDN and, since the account now has a different legacyExchangeDN, the message bounces back and forth between the two Exchange Server computers which are, in effect, saying "Nope-- I don't have him here, you must..." to each other until the message dies when it reaches the maximum hop count.

The legacyExchangeDN isn't supposed to change when moving a mailbox. I'm unsure how it's getting changed based on the information in your post. It's generally frowned-upon to go manually changing legacyExchangeDN's with ADSIEDIT but you could give that a shot (w/ all the usual caveats that come w/ playing with the contents of the directory manually, of course).

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