
Open Mail and allow it to re-index all of your mailboxes.If you are hesitant to delete them, just drag them to the desktop in case you make a mistake and would like the original files. These are the index files that are used to provide references to all of the mailboxes used within Mail. Then navigate to “~/Users/username/Library/Mail/V3/MailData” and delete all 3 files that begin with “Envelope”.
:max_bytes(150000):strip_icc()/006-browser-and-security-updates-for-safari-446556-5f9133263c5949838365627dab916289.jpg)

Right click and select “Show in Enclosing Folder” and note the alphanumeric title at the top of the window if you are doing the search.
#HOW TO UPDATE MY MAC TO 10.11 UPGRADE#
Once El Capitan completed the upgrade process within Mail, it created new alphanumeric folders for each of the old accounts in lieu of the previous naming conventions such as What I didn’t realize is that is also apparently created a new alphanumeric folder that would store the local mailboxes where as previous versions of Apple Mail stored all local folders inside of a folder here “~/Users/username/Library/Mail/V3” called “Mailboxes”. I’m sure there is more than one way to accomplish this and I actually just ran across the “Import Mailbox” solution from within Mail, so there is a good change that may work as well.
#HOW TO UPDATE MY MAC TO 10.11 HOW TO#
Taking a peak into the file structure at “~/Users/username/Library/Mail” it was apparent all of my local folders were still there, it was just a matter of how to get them imported into Mail painlessly.

We finally got around to upgrading to OS X 10.11 El Capitan and ran into an issue where all of my local “On My Mac” mailboxes in Apple Mail didn’t seem to make the transition from 10.10 to 10.11. I am usually pretty slow to upgrade our business systems to a new operating system until there is a good track record of compatibility and to ensure all of our workflows remain uninterrupted.
