Digital signature error could not be decrypted

[wesh44]

Our CIO receives a message with a digital signature, when opened an Error box pops up with "Security Error The message could not be decrypted". You close the error box and can then read the message normally. I have imported the certificate and marked it as Trusted, but the Error box still pops up when you reopen the message or when a new one arrives.

Any help on how to make this annoying pop up go away would be greatly appreciated. Thanks

Groupwise Client 7.0.3