One thing I like with Mutt/NeoMutt is that you can choose what identity to use based on what e-mail adress the message was sent to, something that’s handy for email messages from mailing lists.
I haven’t seen anything about it in the settings, so I assume it’s not possible? If not, would it be something you could consider adding?
If the recipient address matches that of an identity configured in K-9 Mail, the app will use that identity for replies. There’s no generic mapping for recipient address to sending identity.
What has been proposed is to tie an identity to a folder: https://github.com/k9mail/k-9/issues/2155
If you have per-mailing list folders that would probably also work for your use case.
But please feel free to create a feature request on GitHub with your suggestion.