Reject due to invalid SMIME signature certificate

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I have an ongoing issue we've been dealing with for a couple of years. Two of our customers require their employees to digitally sign all outgoing email messages. We've trusted all their root certificate authorities and can get most messages to come in. However, if the employee uses a digital signature AND includes an attachment, the email is rejected with the filter description 'reject due to invalid SMIME signature certificate.' We know they're using a valid certificate and can get incoming emails from the same person in, so long as they do not include an attachment.

I've had open tickets with Oracle for over a year and they have not been able to provide any solutions. We've tried every combination of settings within the mailbox itself, barracuda settings, etc. and nothing has worked.