This means you are trying to send mail via the UTCS SMTP server, but the user name and password is either missing or misconfigured. See our Email Configuration page for details on how to set this up.
Note that the new SMTP server (mail2.cs.utexas.edu) no longer requires you to use a different password from the one you use to retrieve email.
All POP/IMAP users should use "mailbox2.cs.utexas.edu" as their "incoming" server. Note that, in general, CS recommends the use of IMAP instead of POP, especially if you check mail from multiple computers and/or mobile devices.
The SMTP server is "mail2.cs.utexas.edu".
For complete details see our Email Configuration page.
While there may be any number of reasons for the failure, many ISPs and organizations block outbound network traffic on port 25 (the SMTP port) that mail servers use to communicate with each other as one method of stopping spam. If you are using CS's server for your outgoing mail server, we recommend you use port 587 (the Submission port) with STARTTLS turned on. For more information see our Email Configuration page.