Configuring MacMail with an Office 365 McCombs Mailbox

Select Preferences from the Mail menu.
At the next window select Accounts, make sure that Enable this account and Use Autodiscover service check boxes are checked, then click the + button in the lower left corner of the window. NOTE: If you have a previous mailbox set up that you no longer wish to use (such as your old McCombs mailbox), you can remove it by selecting it and then clicking the - button in the lower left corner of the window.
At the Add Account window enter your name as you wish it to appear and then enter your email address and password. Do not enter your username here, but rather your email address. Then click Continue.
At the next window you will be prompted to enter your correct username in the User name field that now appears. Enter it in the format of {username}@mccombs.utexas.edu. Then click Continue once again.
At the Account Summary window ensure that Contacts and Calendars are checked and then click Create.
Congratulations, you are done!