Mail update error upon launch attempt

Corruption of ~/Library/Mail/Envelope Index (which in turn might be caused by some filesystem corruption) is known to cause this bogus error message in Mail, even when there is enough space available on disk — but you must make sure first that there is actually enough space available (a few GB) and that the error message is indeed bogus.

Verify/repair the startup disk (not just permissions), as described here (you may omit this if you’ve already run DiskWarrior):

The Repair functions of Disk Utility: what's it all about?

After having fixed all the filesystem issues, if any, and ensuring that there’s enough space available on the startup disk (a few GB, plus the space needed to make a backup copy of the Mail folder), try this:

1. Quit Mail if it’s running.

2. In the Finder, go to ~/Library/Mail/. Make a backup copy of this folder, just in case something goes wrong, e.g. by dragging it to the Desktop while holding the Option (Alt) key down. This is where all your mail is locally stored.

3. Locate Envelope Index and move it to the Trash. If you see any other “Envelope Index”-named file there, delete it as well.

4. Move any “IMAP-”, “Mac-”, or “Exchange-” account folders to the Trash. Note that you can do this with IMAP-type accounts because they store mail on the server and Mail can easily re-create them. DON’T trash any “POP-” account folders, as that would cause all the mail stored there to be lost.

5. Open Mail. It will tell you that your mail needs to be “imported”. Click Continue and Mail will proceed to re-create Envelope Index — Mail says it’s “importing”, but it just re-creates the index if the mailboxes are already in Mail 2.x format.

6. As a side effect of having removed the IMAP account folders, those accounts may be in an “offline” state now. Do Mailbox > Go Online to bring them back online.

Note: For those not familiarized with the ~/ notation, it refers to the user’s home folder. That is, ~/Library is the Library folder within the user’s home folder, i.e. /Users/username/Library.

You’re welcome.

would it be best to run the combo update before I do your suggestions,
or do these first and then run the combo update?

It doesn’t matter. Installing any system update won’t have any impact on this issue, neither good nor bad.

Thanks! It helped me out too. I don't know what happened, but after I installed the new firmware update, Mail would not run. It opened, and then just froze... odd. Your solution worked for me too though. Thanks again.