CiviCRM Community Forums (archive)

Unfortunately still not able to figure this out.

If you can get the
DEBUG: Send: MAIL FROM:
DEBUG: Recv: 553 5.3.0 ... User unknown
displayed in a browser, just look at the page’s source to see what exactly shows after MAIL FROM: (it’s most probably something like MAIL FROM: <address@example.com>, and you can’t see the address because browsers do not display stuff in <angle brackets>). This is the info you need to take to your hosting provider – this address is not accepted by them, while used by CiviMail as the SMTP envelope sender.

Alternatively, if you know what envelope sender address works, set it at Administer CiviCRM → CiviMail → From Email Addresses screen.