Unable to access an account or mailbox.

Problem

During a migration to Exchange or Office 365, you’re hitting the following error:
Unable to access an account or mailbox.

Cause

Most probably, Office 365 doesn’t know to which mailbox to connect to.
It may happen if the account exists twice — once as an active account and once as a deleted account.

Resolution

  • Login to http://portal.microsoftonline.com;
  • In the administration area, navigate to the user list;
  • Navigate to the deleted user list;
  • Delete the duplicate account.