Like OP it emerged when I had to create a signature for a PDF and naively saved it to my keychain. Apple Mail then began to insist on digitally signing all outgoing mail via entering my keychain password. Following Adobe's advice below to select the errant digital ID wouldn't let me click 'remove.' Adobe's additional advice to go the nuclear option and totally delete keychain and loose all my passwords and other security settings is frankly an amaturish and stupid buck passing exercise of a known issue that others have reported on the forums.
The permanent solution: Open Keychin on your Mac. Select Login from the upper left menu and select MyCertificates from the lower left menu. On the right, you'll be able to see any certificates saved in your keychain. Find the offending certificate: I did this by cross referencing with the expiry date of the certificate I couldn't delete from within Adobe. (Note you need to click the arrow to expand the folder content). Right click to delete it from your Keychain. If you're nervous about deleting it, you could export a copy to your downloads folder first as back up. Now that I've deleted the Adobe certificate, Mail has returned to normal behaviour.