Hello,
K9 Mail with Thunderbird it’s a good idea but for now, I’m using FairMail because is open source and VERY friendly.
For exemple, when I start FairMail I directly see the directory tree (left on attached picture) and I can see the quantity of unread mail by directory (green number)
When I start K-9 Mail I directly see this the mail list (not the best solution for me) and I must to open the directory tree by drag from left Also, the directory list on K-9 Mail is very poor and uncomfortable (the word “INBOX” is repeated unnecessarily.