“Unable to verify message signature : Problem reading digital signature” Apple mail issue
Very quietly Apple introduced a new 'feature' on Apple mail with OS X 10.11.6 that adds 'digital signatures or certificates' to emails as part of the new encrypted emailing facility. It adds a couple of little buttons to the right-hand side of each email window.
Whilst this appears to work OK with single emails if the starburst ballon is ticked, if you use Apple mail for sending strings of emails from FileMaker, it may result in failures of emails with the error shown in the title. This results in all subsequent emails failing and a freeze of Apple Mail requiring an application restart. Simply switching off the signature (X in the balloon) appears to cure the issue.
I have yet to check this process using OS X 10.12 Sierra but I will do this and add to this discussion shortly.