smtp error

Hi, em client has been working fine for some time, but has started to come up with error when trying to send mail. Please help. Tried system restore wondering if settings were changed, but no success…

3:12:42 PM   MailExceptions.ItemOperationException: Cannot send message (‘From’ field address ‘willowfour@southernphone.com.au’ not accepted due to the following reason:

"Rejected - SMTP Auth and Allowed Relays ONLY

If eM Client had been working, I would check my email service provider for what changed.  More likely that eM Client is not at fault here.  Just a thought… 

Thanks, but no, ‘outlook’ on another computer uses same provider which is working fine.

 But, on same laptop ‘mail’ has same issue, not working…same error.

Have found problem in settings, perhaps installing unrelated program changed settings in both email applications, idk…