If you get this error on your plesk server, it means that the server rejected one or more recipient addresses. The server response was:
550 Requested action not taken: mailbox unavailable or not local.
These are the things to check:
- If you have an external name server and have MX records setup externally, you need to swicth off DNS for this domain in Plesk
- The fix is to switch off MAIL for this domain