Have you imported a different certificate in the notification signing certificate of the secure webmail?
For Notification Signing Certificate, the certificate type is an email signing certificate (SMIME). This certificate validates the email notification that is sent to clients letting them know they have Secure Web Mail activity (for example: new message, password expiry reminder, and so on). You cannot use the same certificate that was used for web server certification here; you will need to request an email signing certificate (SMIME) from your preferred certificate authority.
I have managed to find the answer to this question. We have managed to obtain a 25 day s/mime certificate from Symantec. This certificate was downloaded and installed / imported in my browser (Mozilla). Once this was done, I went into the Mozilla certificates section and then backed up the relevant certificate (which was in a .p12 format). I was then able to import this into our Email Gateway and the result was no more error messages or warnings for emails sent by the Gateway itself.