Weird. Dovecot v2.3.31.1 and K-9 (#mozilla Android Mail) have issues. On a new account, K-9 can't find the "Sent" folder. However, works with "Samsung Mail" (which I am assuming is or was a rebranded generic Android mail app?) What is up with that? Even creating an empty directory under Maildir results in K-9 still being confused and not finding a Sent folder.