What OS X version are you using? (Your reference to MobileMe implies your OS X version might not be a recent OS X version.) (If you're not sure which OS X version you're using, > About This Mac will provide details.)
If the account entry in mail is somehow corrupted, you can delete the account, and re-add it.
If you're using a recent OS X version (OS X 10.7.2 or later), re-adding the account will usually be as an iCloud account entry, and an automatic iCloud account setup is available. If it's an OS X version before 10.7.2, then you'll be adding the account and referencing the servers differently.