When you first try to connect to Boxer, it will attempt to autodiscover the account type. Although 99% of the time it does a pretty good job, every now and then it will incorrectly determine an account type. If you're trying to add an Exchange account, for example, and the Gmail login screen pops up, here are the steps you can take to select the correct account type:
1. From settings, select "Add Account."
2. Type in a fake email address (such as firstname.lastname@example.org).
3. This will open up the account type list, and you can select the correct one from here (and enter your correct email address).