This has struck me as an irritating feature of Thunderbird since I switched to it: its IMAP download for offline use only occurs when you switch to offline mode, rather than when messages arrive. This means consciously switching to offline mode each time I move my notebook just in case I do wish to access my email while offline. A much better option (Evolution’s approach) is to to download messages as they arrive…
This can be done by setting mail.server.default.autosync_offline_stores to true.
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