"Unable To Verify Sender Address" or "Sender verify failed" Error Message

If you encounter a "550 unable to verify sender address" or "550 Sender verify failed" error when trying to send an email, it indicates that the MX (incoming) mail server is either not responding, or is non-existent.
Note: these errors do not refer to sender verification (Sending > Verified Senders).

The use of Bounce Notifications will determine if MX records are required and the scenarios include:

• If you have Bounce Notifications disabled, MX records for the sending domain are not required.
• If you have Bounce Notifications enabled and set it to deliver those to a designated email address, MX records are required for that domain.
• If you have Bounce Notifications enabled and set it to deliver those to the original sender, MX records are required for the sending domain. 

We check that the domain has a functioning MX server when Bounce Notifications are enabled to ensure you can receive NDRs (bounce notifications) and delay notifications.

You can check the MX server by searching for the domain on MXToolbox.

If you encounter this error and have further questions, please contact our friendly support team for assistance.