"Unable To Verify Sender Address" Error Message
If you encounter a "550 unable to verify sender address" error when trying to send an email, it indicates that the MX (incoming) mail server of your sender email address is either not responding, or is non-existent.
For example, if you are sending from firstname.lastname@example.org, and you see the above error, it means that the MX server for example.com is either not responding or doesn't exist.
You can check the MX server by searching for the domain name of the sender's email address at MXToolbox.
We check that the domain name of a sender email address has a functioning MX server so the sender is able to:
1) Receive replies to the email that you send.
2) Receive NDR's (bounce notifications) and delay notifications.