Eureka, I found two possible solutions.
The permanent solution: Open Keychain Access 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. Click the arrow to expand folder content. Right click to delete it from your Keychain. If you're nervous about deleting the certificate, 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.
There is also a less permanent workaround in Mail. If encription is turned on in Mail via login keychain settings, there will be a blue star icon below the subject line when you compose a new email. Click that star. It will grey out with a x mark. Mails will send normally, however this might inadvertantly disable any other digital mail signing that you actually do want or need attached to outgoing emails.