Strange Exchange2003 bounces after migrating to new server

Environment I've inherited:
  - Windows2003 domain with file server/users/etc, in domain1 & forest1
  - Windows 2003 & Exchange2003, in domain2 & forest2
No idea why this was setup this way all separate. All users have accounts in both domains, they log into domain1 as primary and their Outlook is configured to point to Exchange in domain2.
To clean up this mess, I've setup a new Exchange2003 server in domain1, and matched up all SMTP addresses from the old Exchange server. I then exmerged all mailboxes to the new Exchange server.

The problem:
For the most part this has been working fine for several days, but this issue is happening to one user's mailbox: replying to any email she sent *before* the migration results in a bounce from the new server, saying 5.1.1 e-mail account does not exist.

I compared her X400 addresses between old/new servers and thought that was an issue, so I added a second X400 address on the new server matching her old mailbox. No difference.

Then I examined emails sent by her from before the move: properties of the "FROM" address looks strange, I see the usual multi-tab properties window but allmost all fields on all tabs are empty. The only data is the DISPLAY NAME field which shows /O=FIRST ORGANIZATION/OU=FIRST ADMINISTRATIVE GROUP/CN=RECIPIENTS/CN=REBECCA

I'm not sure what's hapened, and it looks like a few more users are affected.

This is fairly critical so I'm assigning 500 points. Thanks
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What has happened to the old domain/exchange since you have merged them?
How long have the MX records been changed? (If they have been changed at all...)
Are you using a new IP address for the exchange server?
kd78728Author Commented:
Oh good question, I forgot to mention these. The old server is still alive and MX record is pointing to it still, but its SMTP service forwards to the new server.

There's no issues with sending/receiving external email which is why I didn't mention any of this, the problem is strictly internal. For example, if I reply to one of that user's emails from before the move, it bounces back to me even though my mailbox and hers are on the same server. The only difference is that all her emails from before have an invalid "FROM" address which makes no sense.
Have you tried sending using the Global Address List? I'm assuming this works and you have no problems with that.

I remember something about the reply address containing stamped domain structure information(and for you, this has changed...) but as it's 2am here and I should go to sleep, I seem to be procrastinating instead of going to bed!
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kd78728Author Commented:
Yes, sending using the glabal address list works fine, as does typing an address in the TO: field as usual.

The problem remains that the reply address has changed into that structure. In fact, I looked in the SENT ITEMS folder of those users, and all emails sent before the move are stamped with that same structure as the "DISPLAY NAME" when I look at the properties of any email's "FROM" field. I hope I'm explaining this properly and I make sense.

I didn't spend too much time looking in every mailbox yet, but so far I have 4 users affected by this. But when I look at the properties of the FROM field in their SENT ITEMS folders on the *old* server, it all looks fine. Somehow this field lost its mind during the exmerge process.
kd78728Author Commented:
Anyone got any ideas??
kd78728Author Commented:
Okay so here's an update on what I've discovered:

The users on the 2 separate domains do *not* all have matching Windows login names, causing the Exchange alias to be different also. I move a mailbox to the server, aliases don't match, and therefore replying to an "old email" bounces because the new Exchange doesn't know about the old alias. I didn't even think about this until this morning, about half the users are affected after checking all mailboxes.

Possible ways to fix this, that I can think of right now:

1) Change aliases on new server matching to the old server. Problem with this: replying to emails sent in the interim (between when I moved the mailbox, and when I fix its alias) will bounce.

2) Create an additional mailbox for each affected user with the old alias, hide it from address book, and have it forward to the uesr's main mailbox. I've tested this and it seems to work, but it means maintaning an additional 24 mailboxes.

Is there a clean way of dealing with this??

kd78728Author Commented:
3 days of banging my head to understand this problem so I can find a cure, and I found it by reading the following:

Now that I went through all this, understanding the problem and applying its cure are all so simple-- add X500 addresses for affected users to match their Exchange alias on the old server, with the format:

Time to go to bed...

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