5.2.0 STOREDRV.Submission.Exception:SendAsDeniedException.MapiExceptionSendAsDen

Good morning,
For some reason my automatic emails to myself and customers have stopped being sent. I notice in my Post SMTP log that this seems to have happened after recieving this notification

5.2.0 STOREDRV.Submission.Exception:SendAsDeniedException.MapiExceptionSendAsDenied; Failed to process message due to a permanent exception with message Cannot submit message.

This message is also followed by alot of numbers.

Following this message any emails no longer show as ‘sent’ but have 5.7.3 ‘authentication unsuccessful ‘ messages.

Are you able to help me get these email notifications working again please.

I’m really not very good with anything technical, and am worried about completely messing up my website by messing about with it, so your helpful guidance would be much appreciated,
Thank you