"Failed to send client certificate" error

We have used the email certificate feature previously to send out the Cert generated on the F5, but now we are getting this error : "Failed to send client certificate to ...". I can send a test email and that works, just not a one generated with a certificate.

 

 

We currently have to 4100's (one active, one passive failover) on a DMZ. I admit I'm very green on the networking side of things, but any suggestions on what to test or where to look would be most appreciated.