How to reduce the size of Apple Mail

Apple Mail is taking up 18.64 GB of storage. I went through and deleted at least 2 GB of attachments and unchecked the mail portion of a work account that had probably 3-4 GB of attachments (wanting to keep contacts and calendars). I've done a rebuild on my mailbox and restarted the computer. But the size that it shows it is taking up on the computer (from the manage storage portion of About This Mac) has not changed.

When I go into the Library the folders are all very cryptically named with folders such as 64AF025D-0F66-4CC7-9FAD-D8E0C0E4BE84. It appears that the work account I "removed" from Accounts is still there, as are over 300 MB of a Junk.mbox that doesn't appear to coincide with any of the junk folders in any accounts (which are all empty and the trash has been emptied).

My mail is backed up in iCloud, so I looked at my iMac and the attachments show as deleted there as well, at least indicating it's syncing, which appears to be progress; the iMac showing 15.54 GB, which is about what I would expect from the attachment removal but the 3-4 GB of work email space has not been reclaimed. Do I go in and delete that account and then re-add it with only contacts and calendars?

I'm still mystified at how to get rid of the Junk.mbox and am curious about what the other folders are. I've included some images, but they total up to over 26 GB of space; much more than the shows up in the About This Mac.

Are there any of these folders that I can delete, and if so, which ones? Or should I just delete mail and reinstall it? Any input appreciated.