The domain name part of the sender's SMTP e-mail address is invalid. The domain part, which is the part of the e-mail address ...

{4}: The domain name part of the sender's SMTP e-mail address is invalid. The domain part, which is the part of the e-mail address after the @ symbol, can contain only letters, numbers, hyphens, and periods. Verify that the SMTP address of the sender is formatted correctly and resend the message.