Location of Mail Directories
/Users/Mac_Username/Library/Mail/V2/Mailboxes (actual mailbox)
In MacOS 10.9.2:
(Rebuild messages database)
Location Of Plist Files
Why do I need to know where the .plist files are?
Answer: When they become corrupted and your application starts
behaving improperly, you need to trash them and reopen the
application to rebuild them. All of your customizations will
reset to defaults, but at least you will be able to open the app.