It is good to know where is the default location of Mac Mail stored if you can find out the right file you can use it to restore your emails if you lost your emails. This is actually another kind of easy method to make a mirror copy of your emails. Even if you set your Mac mail client for your Yahoo, Gmail, apple or any other email clients, you will get a single file of your each email inbox as a backup.
Since the release of Mac OS X, Mail.app has been the default email application. Mail stored emails in .mbox files up until the release of Mac OS X Tiger 10.4, at which point Apple changed the default file type to .emlx.
When looking for an email on Mac OS X drive, the standard location for the stored email is in a hidden user library. To view the hidden folder, you have to hold down the option key while browsing on the finder. Please follow the steps.
Click on Finder menu on top menu >Go (drop down menu) hold the option key to see Library ~/Library/Mail
If you think hiding is annoying, you can change in the terminal to unhide, copy and paste:
chflags nohidden ~/Library
Where attachments opened from Mac OS X Mail are stored, or when you open an attachment from Mac OS X Mail, a copy is placed in the “Mail Downloads” folder, Home/Library/Mail Downloads, or the system “Downloads” folder by default. You will find the edited document in this folder.
You can make Mac OS X Mail store attachments on the Desktop. If you want to keep track of files opened from Mail more tightly, you can change the folder used to save attachments and downloads, to your Desktop for example.
Select Mail > Preferences > from the menu > Go to the General category and Select the desired folder under Downloads Folder.