Email error message when sending an email

Hi Team,

I am running windows and Microsoft outlook which is linked to my Main email provider Optus web mail.

I have had no issues up to last week with Microsoft outlook receiving or sending emails as long as my Norton VPN was turned off.   For some reason now I am getting an Error Message   " Email Error:  Your message was unable to be sent because your mail server rejected the message". I can still receive emails but when I send email I get the message. I have no problem of receiving or sending emails on Optus web mail. Can any one help.