cannot change incoming mail server for .mac account

Is this a Mobile Me or iCloud account?

.Mac and .Me accounts try to automatically configure themselves in Mail. This doesn't work very well. To solve this you must force the system into manual configuration.

Delete the account you're trying to work on. Recreate it, but leave the password field blank and select next. This will force manual configuration mode. Select IMAP (NOT iCloud IMAP) enter the incoming and outgoing servers you wish to use. The account will automatically reconfigure itself as a iCloud IMAP account with an entry in the Incoming mail server slot.

Thanks! The manual config trick solved the problem.