All POP 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.
Brand new users (or those who've never used IMAP with their CS account previously), should go ahead and use "mailbox2.cs.utexas.edu" server. Existing IMAP users should continue to use "mailbox.cs.utexas.edu" unless you've been migrated to the new server or have specifically been asked to change it.
The SMTP server is "mail2.cs.utexas.edu".
For complete details see our Email Configuration page.