After taking a cursory look through the archives, I didn't see this mentioned.
I'll name the scenario, then the feature request.

While traveling, we'd like to have access to *all* our e-mail, including all the folders into which we've filtered our mail. We could do this by putting all our folders on the mail server, but our university mail server is restricted to 100MB. My local Mail folder is up to 275MB, and I'm probably low. So, what we'd like to do is run a department IMAP server for all our personal folders, and continue to use the university IMAP server for incoming mail/sent mail, etc, in other words, the small set of default folders.

Feature request:

A user can log into two IMAP servers in one session and move mail across servers. Possible? Doable? How hard?

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