Unexpectedly unable to send mail on ac and iPhone - have tried resetting but no luck

Has anyone ever come across the problem where both iPhone and Mac suddenly stop sending emails despite none of the setting being changed . I have tried checking all configs, I have tried going from POP to IMAP, I have tried deleting accounts and starting afresh.

Connection doctor shows server is connected but keep getting the same message -Verify that you have addressed this message correctly. Check your SMTP server settings in Mail preferences and verify any advanced settings with your system administrator.

The server response was: <****> recipient rejected - relay denied

Select a different outgoing mail server from the list below or click Try Later to leave the message in your Outbox until it can be sent.

Plusnet (Waitrose) couldn't help and blame apple

The frustration is real . any solutions out there ?

[Email Edited by Moderator]