Goto http://webmail.utexas.edu (login with your UTEID).
The New Webmail expands the login screen to include an Express
Login and a Custom Login screen:

Use the Express Login to connect to one of the four most commonly
used servers on campus: mail.utexas.edu, uts.cc.utexas.edu, ccwf.cc.utexas.edu,
or austin.utexas.edu.
Use the Custom Login to connect to all other
servers. Students here at the School of Information can use the Custom
Login to log into our mail server by entering: mail.ischool.utexas.edu in the Server field. Now hit enter or click on Log In. Nothing else is required besides your Username,
Password, and Server. You do not need to change anything in the box
after the Server field (where it reads 143 in the above screenshot),
nor do you need to select anything from the dropdowns below.