Apple Mail has turned into a nice little email application. It is fast, easy to navigate and has a small system footprint.
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Compact database
1. Quit Mail
2. Type or copy and paste this in Terminal:
/usr/bin/sqlite3 ~/Library/Mail/Envelope\ Index vacuum
3. After the vacuum was finished, start Mail
Check the original tip for more info.
Gmail Oddities
Gmail POP access doesn’t work as a normal POP mail server would. Here are some of its idiosyncrasies:
1. Messages downloaded by any conventional POP mail client are marked as downloaded at the server (but not as read on the web), and no longer available to either the same or any other POP mail client.
2. To make already downloaded Gmail messages available for POP access again, you must go to Gmail’s Settings > Forwarding and POP page, select Enable POP for all mail, and click Save Changes, which will cause everything (and I mean everything) still there to be downloaded again. Alternatively, if you only want to download the last 30 days of mail, you can configure Mail in “recent mode” as described in How should I use POP on mobile or multiple devices?.
3. Any Mail > Preferences > Accounts > Advanced > Remove copy from server settings have absolutely no effect on the messages stored on Gmail’s server.
4. Messages created and sent using the web interface will be downloaded by your POP mail client as if they were incoming messages. You can set up a rule in Mail to automatically route those messages to the appropriate Sent mailbox (or to any other mailbox for that matter) if that's what you want. Although this may seem weird, it's good because it allows archiving all messages locally in Mail, regardless of how they were sent -- as long as you don't access the same Gmail account with more than one POP mail client, that is.
5. Messages created and sent using a POP mail client such as Mail are also treated as if they were incoming messages for POP purposes, just as in 4. What prevents them from being downloaded is that Gmail marks them as already downloaded immediately. They would, however, be downloaded if Gmail settings were changed as described in 2 above.
6. As a particular case of 5, messages sent to yourself will appear in your Inbox on the web, but cannot be downloaded by either the same or any other POP mail client.