If you’re seeing the error “The SMTP address format is invalid” in Exchange, it usually means you’ve entered an email address in the wrong format. This often happens when using a domain-style login instead of a proper email address.

Why This Happens

Exchange expects a valid SMTP (email) address. When you enter a username like domain\user, it doesn’t qualify as a routable email address, and the system throws an error.

Where This Error Appears

  • While assigning mailbox permissions via EAC or PowerShell
  • When adding users to distribution groups
  • During mailbox migrations or Outlook setups

How to Fix It

  1. Check the Input – Look for entries using domain\username format.
  2. Replace with Email – Use the full SMTP address: user@domain.com
  3. Validate the Address – Use PowerShell to confirm:
  4. Get-Mailbox -Identity "user@domain.com"
  5. Retry the Action – After correcting the format, try again.

Quick Example:

Incorrect:

Add-MailboxPermission -Identity "user1" -User "exch16adib.com\administrator" -AccessRights FullAccess

Correct:

Add-MailboxPermission -Identity "user1" -User "administrator@exch16adib.com" -AccessRights FullAccess

Conclusion

Always use the email address format (username@domain.com) when working with Exchange settings to avoid this error.