O365 Hybrid Migration Errors & Fixes

This post will go over some of the many errors that occur when attempting to move a mailbox to Office 365. The error messages you see are discovered when showing the details of a failed migration and then clicking on a failed user in that migration.


1. “You can‎’t use the domain because it‎’s not an accepted domain for your organization”

Office 365 will not accept migration of a mailbox if that mailbox has an email alias that includes a domain that doesn’t exist in Office 365. If, for instance, you have an O365 tenant that only has domain.com added to the list of domains and you try to migrate a mailbox with an alias of anotherdomain.com, that migration will fail with the above message. There are two fixes possible for this issue:

  1. Go to the mailbox properties in on-prem Exchange, open the list of email addresses on the mailbox, remove any email addresses with domains that don’t exist in O365.
  2. Add any domains that exist on-prem to Office 365.

2. “The target mailbox doesn‎’t have an SMTP proxy matching ‎’XXXXX.XXX‎”

You can see this error when moving to Office 365 and when move back on-prem. The difference will be the value of the XXXXX.XXX section of the error. When moving to O365, this will be the tenant’s XXXXX.mail.onmicrosoft.com domain. When moving back on-prem this will be some other domain.

On-prem to Cloud

For moving to cloud, this issue will usually occur because the user is not applying email address policies. The email address policy is modified by the Hybrid Config Wizard to include a policy that adds user@domain.mail.onmicrosoft.com to each user’s proxy addresses. If users are not set up to apply email address policies, they won’t get that address added automatically and it has to be added manually.

Cloud to On-prem

When moving on-prem, the error pops up because of an incorrectly configured migration batch. The cloud to on-prem migration will prompt for a delivery domain. This domain *must* match a domain that the user has as an email address. It must also be a publicly usable domain. So if you choose the domain.local delivery domain for the migration batch, it will generally fail because that isn’t a valid public domain. In addition, using domain.com as a delivery domain will fail if the user doesn’t have a proxy address that matches that domain.

Continuous Updates

I’ll be updating this page as I discover new migration error messages. I’ll have to see the error message before I can post the fix here, so if you have one that isn’t listed, let me know in the comments and I’ll add it to the page.

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