Receiving works, sending fails. A short list covers almost every case.
1. OUTGOING AUTHENTICATION
It must be enabled, with the same username and password as incoming. This is the cause most of the time.
2. THE PORT
465 with SSL, or 587 with STARTTLS. Many networks and mobile providers block port 25 entirely.
3. THE USERNAME
The full email address, not the prefix.
4. THE FROM ADDRESS
It must match the mailbox you are authenticating as. Sending as a different address is refused as relaying.
5. SENDING LIMITS
If you have hit the hourly limit, sending pauses until the hour turns over. Check whether you or an application on the account has been sending heavily.
6. THE RECIPIENT
"Relay access denied" is your own configuration. "User unknown" or "mailbox full" is the recipient's.
7. YOUR IP
Repeated failed logins block your address at the firewall. If nothing works from your network but does on mobile data, open a ticket with your address.
