A poster on the macOSXHints forum wants to know 
how he can delete all the account information, emails and files that Mail.app uses, so that he can use Thunderbird in peace without wasting disk space.
He is told that he needs to delete the ~/Library/Mail folder. That’s the biggest disk hog, but it is not the only one.
In 10.4.x, Mail.app also sometimes stores attachments in a folder called (by default) ~/Library/Mail Downloads. You could kill that to save some space.
If you are in the mood, why not carry on and delete the Preferences file (com.apple.mail.plist) which stores all your account settings and so on. It’s in your ~/Library/Preferences folder.
For the final purge, you could delete Mail.app itself (46.8 MB) from the Applications folder. A desperately sad business.
Personally, I leave everything outside my home directory untouched, because I don’t know how a security or point release updater would react to things not being where they are supposed to be.
On my computer, I run solely as a regular (unprivledged) user. The only third party applications that I run are the ones that allow for drag and drop installation, which I always place in ~/Applications. I refuse to run applications that require installers.
[BTW, ~/Applications is a recognized spot for applications. If you create a folder called "Applications" in the top of your home directory, it even gets a system icon.]
When in doubt:
http://appzapper.com/
Does a pretty good job:)
jeff.. you are a very boring person. live a little…