Currently I have an HTML signature for my work account on Thunderbird desktop that contains some simple text formatting and a logo image, but pasting the same HTML into the signature setting in K-9 Mail just inserts the HTML as text into the email.
This is actually a stunningly important feature that a lot of mail clients miss. Every organization I work for uses HTML signatures, and I've yet to find a mobile client that will handle that gracefully. If K-9 had the same functionality as thunderbird that would be a huge relief.
I'm looking exactly for the same. I hope Mozilla team would like to update K-9 Mail to support exactly this feature to have similar mail client as Thunderbird