outlook

Clarifications

Cert errors happen most often when the time/date is waaay off on the PC.

Have you inspected the cert to see if it is valid?

Maybe some malware is doing a DNS hijack and confusing the cert process...

Does it work when the client installs the cert?

I've seen that error pop up on email systems that use smart cards. If access to the email account is managed by the smart card, and you try to open an old email that has an old certificate (ie, the certs are expired, or someone on the email is no longer in the system), it pops up the warning. You should be able to just manually clear it, unless you're trying to automate something via Macros...then it's a matter of putting in some error handling to identify the specific email that's throwing the error and processing it manually.