Outlook reports a security problem with an encryption certificate

When I attempted to send a PKI-encrypted email message to several recipients from Microsoft Outlook for Mac 2016 (version 15.41), I saw the message "Encryption certificates for some recipients could not be found in the Outlook Contacts. Do you want to search the Active Directory servers for them?"

Encryption certificates for 
some recipients could not be found in the Outlook Contacts

I clicked on OK and then saw the message "The following recipients have encryption certificates with security problems. Do you still want to send this encrypted message?"

Outlook - encryption certificate
with security problem

I clicked on Cancel and checked on whether Outlook was storing a public key certificate for the person in my contacts list. He was in the contact list, but there was no certificate associated with the person in the contacts list when I clicked on the Certificates tab.

Contact with no certificate

I deleted the contact anyway to see if that would eliminate the message about the issue with his security certificate, but when I tried sending the message again, I received the same message. I closed and reopened Outlook and tried sending the message again, but though I no longer received the message about some recipients having missing certificates, I still saw the message about there being an issue with this person's X.509 security certificate.

I then opened the Keychain Access application in the Applications/Utilities folder on the MacBook Pro laptop, but when I clicked on Edit and Find and searched on the person's last name, no entries were displayed. I did find some Basic Encoding Rules (BER) .ber files for security certificates in the root directory of the hard drive, i.e., on "Macintosh HD", but deleting those didn't affect the problem. I still saw the same message after closing and reopening Outlook after I deleted them.

I then logged into the Entrust Entelligence Secure Desktop 8.1 application on the laptop and clicked on Find Recipients and performed a search on the person's name.

Entrust Find Recipients

When I saw an entry for the person appear in the "Recipient DN" list, I clicked on that entry and then on the Add to Keychain button. I then saw an entry for the person in my keychain in the Keychain Access app. When I reopened Outlook, I was then able to send the message to everyone in the "to" and "cc" list without any warning message appearing. I then added the person's contact information back into my Outlook contact list; there's no certificate associated with the contact entry..