The email files that contain encrypted messages are known as p7m files. In most of the cases the sender sends an encrypted key to the receiver so that the receiver can open the message. 

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Software to open a p7m files  Silky Terrier Dog Breed Playing Around

It’s very difficult to decrypt the private keys but they are some application that help in the decryption process 

1. P7m viewer
2. Sec Signer 
3. Digi Signer