[wiz21c]

Using claws-mail since 3 years, works really well (but I'm connected to the internet at all times, dunno how it works when out of connection)

[Fnoord]

If you're offline a lot you can use OfflineIMAP [1] or say rsync your Maildir.

Something like Mailspring (successor of Nylas N1) also allows for sending an e-mail later during the day.

[1] https://en.wikipedia.org/wiki/OfflineIMAP