Hi Sarfaraz,
thank you for your response.
I Answer your questions:
1 - PGP Viewer normally decrypts the e-mail.
2 - The popup window shows: "Incoming Email. Result - Not secured by sender"
Although when sending the message was "encrypted Email".
3 - MAPI (MS Exchange client).
I think one of the latest MS updates broke the functionality of the decryption built into MS Outlook 2013.