Why can't I send mail to a certain domain?
- They are blocking email from your IP address, or your domain.
- The mail is being sent to them with a "reply to" of a null sender, or a malformed email address. (Some server admins do this to try to block spam).
- The mail is being sent to a generic name like "unknown recipients", and the real recipient is listed in the BCC. The destination server could be blocking this because it could be junk mail.
- The destination server is trying to verify if the sender email address is valid (again, trying to block spam).
- The domain has bad DNS MX records.
- The destination server is down.