Can p7mViewer decrypt all encrypted messages?
No, p7mViewer can read only those encrypted messages for which you own an appropriate private key. Each encrypted message has been encrypted to one or more recipients (and probably to the sender). Each of the recipients can use his or her private key to decrypt the message. Thus, you need to be the owner of one of the private keys and have them installed in the system. Additionally, p7mViewer can use PKCS#12 files (.pfx or .p12 files) without installing them.
