The error "550 Sender verify failed (or 550-Verification failed for <email address>)" can occur when SyncBack connects to an email server using an email address that is not present or active in the server.
So, ensure the email account you are using to send emails (under Log > Email Log settings page for the profile) actually exists in the server, and that you are using the correct username and password for authentication. If you are unsure of the sending server connection details, please check with your system administrator or use a different email account to resolve the issue.