The 'recipient address rejected: access denied' error message generally signifies that the receiving mail server is refusing to accept the email. Common reasons include the recipient's email address not existing or being invalid, the mailbox being full, or the sender being blocked. A 'no route to host' variation from Microsoft MX indicates a networking problem preventing validation of the address, possibly due to internal MX issues, a discontinued domain, or DNS caching problems. If other addresses in the same domain are delivering successfully, it strongly suggests that the specific recipient address is no longer active.