You’re migrating emails to an Exchange server or to Office 365 and the message “Error writing mail (code 3): The primary SMTP address must be specified when referencing a mailbox” is displayed in the migration logs.


This error occurs when the target email of the user references an alias instead of the primary SMTP address.


  • Update the user’s target email in Cloudiway Migration and set it to the primary SMTP address.
  • If the email address in Cloudiway Migration is the correct email, verify in Exchange that the primary SMTP address is the correct one