My SMTP server requires a username and password. Where do I configure these values so I can use the built-in 'Send Mail to Contacts' task? I get a fatal error: "authentication failed" when trying to submit a contribution.

If you are getting an "authentication failed" fatal error when trying to "Send Email to Contact" OR at the end of attempting to submit a contribution with CiviContribute, you need to configure your SMTP settings for SMTP Server authentication.

Begin at the Administer CiviCRM page. In the Configure section, choose Global Settings, then Outbound Email (SMTP). SMTP Username: Enter your Username here. You must also enter you SMTP password in the CiviCRM settings file (civicrm.settings.php).

See Outbound Email (SMTP) for more information.