trash.mbox taking lots of disk space

I had the same problem. Try this, it worked for me. First, log into the gmail web interface and clean house, deleting trash, etc. Then, go to mail.app and synchronize all accounts. In the sidebar of mail.app expand the gmail account and locate the trash folder (this may be equivalent to the trash folder at the top of the sidebar, above "On my mac", I'm not sure.) It should be empty, reflecting the status of the trash folder on the web interface you just emptied. But, if you are like me, your [gmail].mbox:trash.mbox will still be huge. So, highlight the empty trash folder in mail.app and rebuild it. You may need to repeat the rebuild process a few times. You should notice the size of trash.mbox decrease significantly as the rebuild progresses. Mine decreased from 20 Gb to 300 Mb. In fact, I ran rebuild on all my mail folders just for good measure. Not sure why this is necessary, but it looks like large numbers of deleted messages and drafts get orphaned in the trash folder and a rebuild is required to properly sync it with the trash folder maintained on the gmail server.

Experiencing the same issue for multiple Gmail trash boxes on my Mac (OS X 10.9.4), to be honest, I chose the straightforward way and deleted the directory of trash files under >Library>Mail>V2>IMAP-...@imap.gmail.com>[Gmail].mbox>Trash.mbox, in order to keep the info.plist file only. Of course I did it after having quit Mail. Worked like a charm.

Mail is known for having a lot of problems since OS X 10.9 was released, in particular with Gmail boxes. Large Gmail boxes are not well handled and we all look forward to having these issues fixed in Yosemite, it was by the way a commitment from Apple leadership team during the last developer conference.

Let me know it worked for you.

Best,

Thiebault

I also first closed Mail, cleaned out my Trash on gmail, then deleted all of the files under the Data folder below my Trash.mbox and restarted Mail. All this worked nicely and I was rewarded with 38+ GB of free data back. But I think that I also fixed a problem with Mail starting slow or taking a long time to see new mail from Google or taking a few minutes before being being able to send mail when I move from one network connection to another. I am not 100% sure this truly fixed the problem - but in general Mail seems a lot snappier with 38GB of Trash gone from my HD. Thanks for the thread.

Try Mail/Preferences/Accounts/Mailbox Behavior - you should be able to set a automatic delete there. If you can, quit Mail and reopen and your trash should be emptied.