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).
