You can set up your Mac so users can share the same main Mail directory by following the steps below.
Copy Mail to Shared Directory
Select which user’s Mail setup you want to use as the basis for all other accounts then copy their Mail directory to the Shared users folder.
Note: This tutorial uses the username “userone” for the primary user (whose email you are using as the basis of your shared directory) and “usertwo” for a secondary user who will also be accessing the shared mail directory.
Open Terminal and enter…
sudo cp -vR /Users/userone/Library/Mail/ /Users/Shared/Mail/
Set the file permissions so all users can access them along with all new files that may be added.
sudo chown -R userone:wheel /Users/Shared/Mail/
sudo chmod -R +a "wheel allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit" /Users/Shared/Mail/
Backup and Create Symbolic Link
Backup each users current mail folder and then set up symbolic links to the new shared mail folder. (Note: there is no trailing slash after the symbolic link declaration)
sudo mv /Users/usertwo/Library/Mail/ /Users/usertwo/Library/Mail-BAK/
sudo ln -s /Users/Shared/Mail/ /Users/usertwo/Library/Mail
Test and Cleanup
Test by opening up Mail.app for usertwo and verify it’s working as you expect. If so repeat the two commands above for all other users that will share that Mail folder (including userone).
After you have set it all up and used it a bit you can go back in and clean up all the backed up mail folders (repeat for all users). I recommend waiting a couple days just in case you run into any bugs and need to revert. Leaving the backed up folders there won’t hurt anything aside from taking up unnecessary harddrive space.
sudo rm -rf /Users/usertwo/Library/Mail-BAK/
Rebuild Search Indexes
You will likely also want to rebuild your indexes. The most efficient way to do so is to go in and remove your index files.
rm -rf /Users/Shared/Mail/V2/MailData/Envelope\ Index*
Next time you open Mail it will tell you it needs to import your messages. Press “continue” and wait for it to complete. Once done you should be back up and running.