Is there any difference, currently? Aside from the logo, I can’t see anything different from K-9 Mail.
One of the biggest draws is the ability to transfer from K-9 Mail to Thunderbird for Android
I literally just tried that, and it failed. LOL. Ended up setting up the account manually.
Edit: Also doesn’t seem to have the push mail option like K-9 has. It does have push, it’s just enabled very differently. Go into Folder Settings, select a folder (Inbox in my case), and then turn on “Enable Push”. You may also need to grant it Alarms and Reminders permission manually on newer Android versions if it doesn’t prompt for it.
See edits: Push is present and totally works; enabling it is just very different from K-9
On K-9 Mail, it’s in Settings -> {Account} -> Fetching Mail -> Push Folders
On Thunderbird Beta, the “Push Folders” option should be in the same place, but it is totally missing for me.
Edit: If you go into Manage Folders and then Inbox (or any folder I guess), there is an “Enable Push” option. I enabled that, but it doesn’t seem to do anything.
Edit 2: Ok, the folder-level enable push seems to work now. I had to grant the Alarms and Reminders permission before it could run in the background. It didn’t automatically get or prompt for that, but granting that in app settings fixed it