You want to look into asymmetric encryption. This type of encryption allows you and you only to decrypt messages.
The encryption has a few processes to set it up:
1) Generate a public key (to give to other people)
2) Generate a private key (for you to encrypt messages)
This allows people to encrypt messages and send them to you and only you can decipher them. Since this process is one way (sender encrypts, receiver decrypts), this means that the other party would have a similar system in place for you to encrypt messages.
So here is a basic process. Involving Alice and Bob.
1) Alice creates a public/private key and sends the public key to Bob.
2) Bob creates a public/private key and sends the public key to Alice.
3) Alice wants to send an e-mail, so encrypts using Bob's public key and then sends the message.
4) Bob decrypts the message with his private key.
5) Bob replies by using Alice's public key.
6) Alice reads the messages by decrypting using her private key.
Therefore, you can have an e-mail of this format:
--START PUBLIC KEY--
--END PUBLIC KEY--
I hope this helps,