On Linux I used Astroid for offline mail access (notmuch + lieer for gmail and mbsync, I think, for other mailboxes - it worked so good that I don’t remember exactly). For me it is the best offline email setup.
On macOS, unfortunately, I haven’t found anything better than Apple Mail, considering the integration with OS. There are downsides, though. Offline format is incompatible with everything else, it’s hard to do rsync backups of ~/Library/Mail, and I don’t think all mail is accessible offline. It is slow, and it mangled non-latin attachment names for Gmail letters. On positive side, there is a great support for drag&drop, Spotlight integration, multiple account handling.
On macOS, unfortunately, I haven’t found anything better than Apple Mail, considering the integration with OS. There are downsides, though. Offline format is incompatible with everything else, it’s hard to do rsync backups of ~/Library/Mail, and I don’t think all mail is accessible offline. It is slow, and it mangled non-latin attachment names for Gmail letters. On positive side, there is a great support for drag&drop, Spotlight integration, multiple account handling.