I mentioned last year that there’s an easy way to see how large all of your mailboxes are, which is useful if you’re trying to figure out where best to start cleaning up old emails. Unfortunately, El Capitan’s version of Mail has done away with the little gear icon that I pointed out in the last tip, so with that gone, what do you do? Well, luckily enough, it’s still simple. Now you’ll need to right- or Control-click on a mailbox and choose “Get Account Info” from the contextual menu instead.
When you do that, things’ll be similar to what you saw under previous versions of the OS—you can switch the account you’re looking at, say, from the drop-down at the top, and the mailboxes for that account will appear near the bottom with their message counts and their sizes (which could take a while to come up if the mailboxes in question are huge).
And as I mentioned in the article I linked above, if you need to archive messages out of Mail entirely (or just into an “On My Mac” folder), there’s a separate tip I wrote about how to do that, too. Hey, not every single email host gives you oodles of space, so sometimes you just gotta bite the bullet and clean stuff up, I guess.