In Troubleshooting Apple Mail in OS X 10.11 El Capitan, a user asked:
My question is will the Mboxes import back to Yosemite after being used in El Capitan, what I mean is can you go back to V2 from V3.
The short answer is, yes, if you export your mailboxes in Mail, you can import back into Yosemite Mail using File > Import > Apple Mail.
However, you must revert OS X 10.11 El Capitan to OS X 10.10.5. Reverting is not an easy process. Many files that upgraded like Mail from V2 to V3,will also have to be reverted to previous version. See Apple’s KB: Revert to a previous OS X version using Time Machine.
I would advise users that are having issues after upgrading to El Capitan, try to resolve their problems than try reverting. See Before upgrading to El Capitan Mail for tips that will help to nuke your failed El Capitan Mail upgrade.
Some users have opted to move to a new User rather than fix the issues in their current User’s folder. You can test Mail first in a new User to verify the problem is in your User’s folder and not with your OS X install files.
You can use the Guest User or create a new User for testing. When you log out of Guest, all data entered will be erased. I suggest creating a test user in case you end up moving to the new User or simply want a clean environment to test future issues.
First grab your email settings and passwords. You can save these to the Shared folder for access in the new User.
Your Hard Drive/Users/Shared
CREATE A NEW USER
Go to System Preferences –> Create a New User in Users & Groups.
Switch to the New User by logging out/in or use Fast User Switching.
You can skip the Apple ID setup unless you are testing iCloud.
Select Don’t Sign in > Continue > Skip
Only default Apple apps will be in the Dock in the new User. Go to Applications to open other apps you might want to test.
Open Mail and add your accounts. I would add one at a time and let it download from the server before adding additional accounts.
Note: Any POP mail you download in the test User will not be available unless you set to “leave on server”. Under Advanced tab in Accounts, do not check the box to remove copy from server…
If you are happy with Mail in the New User, make sure it’s set for Admin status. See this article to help moving your other data to the New User.
Apple includes an export option under Mailbox in the Menu bar. This will export your data as an Apple mbox file. This is not a standard .mbox file. Only Apple Mail can import this type of .mbox file. This .mbox will import into any version of Apple Mail.
You do not need to export your IMAP or Exchange folders. You simply add the accounts back and let the messages download from the server.
You do need to export your POP accounts (Inbox, Drafts, Sent, Trash* and Junk*) and any mailboxes (folders) under “On My Mac”.
Select all Mailboxes you want to export. You can select all the top default items (Inbox, Drafts, Sent, Trash, Archive, Junk, VIPS, Flags.
[Tip: Export each account’s default folders into a folder with account name. Otherwise, if you select all Inboxes you’ll get folder names like INBOX, INBOX 1, INBOX 2, etc.
Do not just select the top Unified root folder. Expand to show all Inboxes, Drafts, Sent etc.
Under Mailbox select Export Mailboxes…
Choose a destination folder. Be sure to click on Options to get the box: Export all subfolders.
After exporting default items, select all the local “On My Mac” folder. Be sure to include subfolders.
Note: you can manually expand all folders to view subfolders. Select all in the expanded view to export.
Note: You can export Smart folders, Flags and VIPS like a regular mailbox. Smart folders with MailTag keywords caused Mail to crash for me.