You can access multiple mail accounts with OS X Mail v 5.1 with ease. Also, deleting mail accounts already added in OS X Mail is equally easy. In fact, I feel that nothing could be easier than deleting the OS X Mail account(s).
To delete an unwanted Mail account in your Mac, locate the Mail application in your dock or in the Application folder and launch it. Once launched, click Mail located next to the Apple logo and then go to Preferences (Mail -> Preferences). Then go to Accounts tab and all the accounts added earlier (whether enabled or disabled) show up in the left pane. Click to highlight the account you’d like to delete and press the minus “-” button located at the bottom of the Preferences pane as shown below.
You’ll be asked to confirm if you want to delete the mail account. If you’re sure go ahead and the mail contents will get deleted.