This issue may not be caused by the certificate (public key) itself.
Thanks to forum ID 'Griff' for this information.
Below is one possibility:
In this case, Thunderbird was the receiving client.
- The receiving email client would not recognize that the incoming email had a digital signature attached, and therefore couldn't/wouldn't add the public key to the Certificate list.
- The message source indicated that the digital signature was indeed in there, but it did not look quite right.
- The email server in question used DSPAM for spam control, which inserts a Token into the email body showing its Dspam Verification# in the email.
- The insertion of the DSPAM Token corrupts the Digital Signature information, which is why the receiving email client couldn't/wouldn't interpret it properly.
- Reconfigured DSPAM (this is a server-side config only) to insert the DSPAM Token into the HEADER only rather than the email BODY.