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.
Hello - I can't migrate to K-9 or Thunderbird Mobile until I can have a signature that includes images (jpg & png), and QR codes in it, as well as text and web links. I am a freelance writer/performer, and I have to be able to include latest tour dates for show, with booking links and festival logos etc.
I am currently using Edison Mail (free version) on Android, and I can email the signature block from my desktop client (Thunderbird) and just cut and paste it into the relevant account on Edison Email, and it works fine.
This, an html signature, is the biggest issue, which prevents my using the mobile thunderbird/k-9 email for business use, and hence personal use. It's a necessary feature to implement if a wider adoption or a more serious use of your mobile client is sought. Unless, some form of non-html text formatting could be implemented.