That is interesting because I currently have 8 email accounts with only 4 of them "enabled" in the Mail preferences.
When I go to the new mail>from drop-down box I only see the four accounts that I have enabled.
So, you may have a damaged preferences file.
1) Double check that you have deleted the old accounts in the Preferences>Accounts.
2) Make sure you have deleted the old accounts' outgoing mail servers from your servers list in Preferences>Accounts>Outgoing Mail Server>Edit SMTP Server List
3) Try using disk utility to repair the drive and repair permissions on the off chance that a file is damaged and can be repaired. Shut down all other applications first.
4) If these fail then you could delete the preferences plist file from your preferences folder. But this may require you to re-enter some of your information in Mail. (Try just moving the plist to the trash first though with Mail not running, then re-open Mail. That way you can move it back if it does not have the desired result.)