Can someone unpack this post for me a bit? I wasn't aware Mavericks expanded email data so much?
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There doesn't seem to be much detail about Mavericks Mail, except people are unhappy about gmail. The original post outlined my particular version of the problem. I don't know if anyone else in the world has this issue, but it became quite a problem for me.
X.9.1 seemed to cut down on the massive internet usage, although I regularly get (for example) "Downloading 1459 of 1459 messages." or "Saving 49 messages." (while composing), in my Mail Activity window. Just not the extra 200GB of data a month, I'd been getting with X.9 and the first Mail update.
I trust Apple will still be working out the bugs in Mavericks Mail, and not resting just yet. I still see it (re-?) downloading years old emails, that appear fleetingly in Notifications banners, but not in the list when I click Notifications.
I enjoyed Postbox for a while, but it didn't seem to solve the problem completely, which makes me wonder what caused the issue. Some underlying mail daemon or something??
As for changing to Postbox permanently, I missed too much of Mail's functionality. Postbox had some good ideas, but (for example) one click to preview a hyperlink within Mail, is just too vital to my workflow, now.
I'm happy with Mac Mail, generally, but it is prone to developing major issues with major system releases. No idea why. It just does.