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I don't know the answer to your first question, but in response to your second question, yes, formatting is preserved.

If you're a technical sort, offlineimap is quite a good tool, although I don't think it compresses downloaded emails.

To answer your first question: It's certainly possible from a technical level, but I'm not aware of any end-user application that enables this functionality out-of-the-box.

One hacky-but-straightforward way to do this would be to take advantage of an email<->filesystem translation like NMH or MBox, and then write some script that would automatically purge the attachments before archiving.

You'd still have to download the attachments, I believe; I could be wrong, but I don't think IMAP will let you get around that easily.

(To be fair, Gmail's implementation doesn't adhere to IMAP very well in my experience - labels/folders/tags are a major problem - so YMMV).