I can't login to my Comcast email

I still don't understand the one mailbox at a time thing, but I think I have found the problem. It turns out I am correctly entering the password and all the information, but because I had "automatically detect and maintain account settings" box checked, any information I put in my Accounts dialog is automatically changed to some previous information! I checked this by putting in a short random password, then closing and saving the dialog. When I open it, a longer password has been inserted.

I even got everything to work by unchecking automatically detect and maintain account settings" and entering my correct password, and my emails started to populate. But soon it stopped working and started asking me for my password again. When I checked the dialog, "automatically detect and maintain account settings" had become rechecked! In fact, every time I uncheck this box, it comes back checked when I reopen the dialog, and puts my old password (I guess) in the password field.

So I need to know how to a) keep this box unchecked or b) if it is checked, make it fill out the fields in the dialog with the correct data.

How does this "automatically detect and maintain account settings" feature work, anyway? Where is it getting the information, in particular the old password, that it uses to fill in the fields?