As an IT Admin, mailbox migrations are a constant thing. Moving users to new databases because you have a new database or you trying clean up and old one with few users, or simply moving to Microsoft 365, there are times when things error out.

Errors are not always related to back pressure or space on the servers or some other form of resource constraint but can also be due to incorrect configuration.

If you prefer to perform mailbox moves through the Exchange Admin Center (ECP), you need to do a bit of navigating to get an actual error. In the example below, an Admin went and set Mailbox Size limits on mailboxes themselves without checking that the database has size limits enabled.

This caused some mailboxes to fail with a quota error but the error only presents itself a bit later as the EAC takes longer to work with moves and display error messages.

If you prefer to use PowerShell, you will see the error almost instantly as shown below:

Because of a simply mistake it took a bit of time to find all the mailboxes that were set manually and set them back to use the database defaults defined.

Once the changes took effect, the command was run again, this time not giving an error and proceeding with the move as shown below:

Always check your moves and don’t assume they will always just succeed. You may encounter a similar error as shown above or the others like back pressure etc.

Hope it helps.

By edward