If you receive this message when sending email via your email client (eg outlook) then check its settings and modify as below:
Outlook
1. Click Tools and E-mail Accounts ...
Two. Select "View or change existing e-mail accounts" and click Next
Three. Select the account and click Change
4. Click on "More settings"
5. Select "Outgoing Server" tab
6. Enable the "My outgoing server (SMTP) requires authentication"
7. Select "Use same settings as my incoming email server"
8. Click Ok
9. Next
10. Finish