Moving locally stored mail to server storage in Apple Mail

Objective

Move emails stored locally in Apple Mail on your Mac to web server storage. This is especially important to do before updating to macOS 10.15 Catalina. At this time, these emails may be lost at the time of upgrade to Catalina.

Environment

• macOS
• Apple Mail

Procedure

1. Open the Mail app.
2. Locally stored mail files are located in the "On My Mac" section of the Mailbox listing as shown here. Click on the local mailbox:
   

    
3. Once in the local Mailbox, go to the top menu bar and click "Edit → Select All" to highlight all mail in the mailbox.
   
4. Right click (or two finger click) one of the selected emails and then in the drop down menu, select "Move to" and the "Inbox" of the account you want to move the emails to. The transfer time will vary based on the number of emails that have been moved.
   
5. The bottom left corner of the Mail window will show the progress of the transfer.
   

    
6. Once complete, the email will be available from the email server and will no longer only be stored locally on the Mac.
7. Repeat these steps for each folder that is in the "On My Mac" section.

Additional notes

Alternatively, you can move a whole MailBox folder as described in Apple's "Move or copy mailboxes in Mail on Mac" article.