You can check first:
- Make sure your email address is setup correctly.
- Check that all characters in your username and password are correct (capital where capital is needed, and lowercase where lowercase is needed)
- Make sure your login is your full e-mail address firstname.lastname@example.org
If it is still not working you can change the incoming and outgoing mail servers to your computer's IP address instead of mail.domainname.com. The IP is located in your Plesk control panel (ex 22.214.171.124)