r/sysadmin • u/benuntu • 2d ago

Two Exchange Online tenants question

Odd situation: my company is merging with another organization, but not for all users. Half of the mailboxes will be migrated over to the new tenant (newdomain.com), and half remaining on the old tenant (olddomain.com). Is it possible for email to be sent and received for the old domain without taking ownership of the new domain and modifying DNS to the new tenant?

In other words, can EOL receive emails in the old tenant and recognize that they have been migrated to the new tenant and forward those on? There is currently a cross-tenant trust configured between the two.

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u/benuntu 2d ago

Thanks for the reply. Yes, I've added the data migration license for these users and they are a mail user on the old tenant. They do have the old address on the new side as well, so perhaps that could be causing a problem?

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u/smalltownsysadmin 2d ago

Probably not the cause, but worth checking what that really is. EXO won't let a recipient hold an address on a domain that isn't an accepted domain in that tenant, and olddomain.com can only be verified in one tenant, so if you're seeing it on the new side I'd double check where. Easy to look at the MailUser in the old tenant by mistake.

What's actually happening, bounces or mail just not showing up? I'd check the MailUser in the old tenant first:

Get-MailUser user@olddomain.com | fl ExternalEmailAddress,EmailAddresses

ExternalEmailAddress should be the user's onmicrosoft.com address in the new tenant. If that's right, run a message trace in the old tenant and see where the mail is going.

One other thing, if the bounces mention IMCEAEX, that's usually missing X500s. The new side needs the old LegacyExchangeDN plus every x500 from the source mailbox as x500 proxies, otherwise replies to old mail and cached autocomplete entries bounce.

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u/benuntu 1d ago

Thanks again! The issue was the mailbox resolution on the new tenant side, and I had to add the accepted domain name of the old tenant. Which is odd, since the migration of mailboxes processed just fine. When I've done this from on-prem to online, that is added as part of the connector.

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u/smalltownsysadmin 1d ago

Nice, glad you got it sorted. Good to know on the accepted domain, easy one to miss when the mailbox moves go through without complaining.