A mailbox failed to be migrated to Exchange Online with the following error:
SourceRecipientDoesNotExistException: The source email address firstname.lastname@example.org couldn’t be found in the on-premises domain.
The mailbox is being migrated from Exchange 2007 to Exchange Online via a staged migration.
This is the only mailbox that has failed so far with this error (out of approx. 150 successfully migrated mailboxes). The AD account is disabled, and the mailbox is hidden from Exchange Address Lists.
Unhide the mailbox from Exchange Address lists (uncheck “Hide from Exchange address lists”). This clears the msExchHideFromAddressLists: TRUE value from the AD object.
Resume the mailbox migration and it will start successfully.
The mailbox can be hidden from Exchange Address Lists again once it’s been migrated to Exchange Online.