Cleaning out my old emails. Circular process.

Gmail really doesn't like to delete mail entirely through the simulated IMAP interface.

You might try accessing Gmail though their web interface, going into All Mail, and asking it to delete the messages.

By default, you cannot delete IMAP messages in Gmail. This is the way they've set things up. Presumably they want to keep your emails around so they can better target advertisements to you.

You can change this setting if you log onto the Gmail web site. Go to settings, then IMAP and POP, and you'll see these:

When I mark a message in IMAP as deleted:

When a message is marked as deleted and expunged from the last visible IMAP folder:

You need to turn Auto-Expunge to OFF, and then change the second setting to "move the message to the Trash."

Then it will allow you to delete things via IMAP.

A word on this: I found that when I did this, messages I deleted kept reappearing in my inbox. Presumably Mail is not issuing the expunge commands as quickly as Gmail likes it, so it puts them back in the inbox. This happened to me enough (10+ times over a few days), that I reverted this setting.

The main setting here I think is to hide the "All Mail" folder in Gmail's IMAP settings, i.e. set it to not show in IMAP. Becuase if it's shown in IMAP, and you don't have the settings as I've shown above, your email will always be visible in Mail.

I'm really not a fan of how Google has implemented IMAP with Gmail. It is horribly nonstandard.