You have to setup PGP on the client side and use it to decode the messages. Using PGP is a bit tricky, and it really does help if you develop at least a basic understanding of encryption. Basically, you need to setup public and private keys on both server side and client side; the program on the server will encrypt the message to you (using the client side's public key), which can then be decoded using the private key and your passphrase. Most email clients don't have built in PGP support - you'll generally need a plugin. Keep in mind that if you're receiving these emails on your personal computer, it's important to keep that machine secure as well - someone who compromises this machine could log your keystrokes and thus obtain your passphrase as well as your private key.
You might want to take a look at:
This is not an "easy" solution, although it is probably easier than some of the other solutions for this sort of thing. Depending on what you're trying to do, you may want to see if there's some sort of outsourced / third party solution which will let you do what you want without having to deal with all of this stuff.
You'll also want to make sure that the script you're using can't be abused by spammers to send messages through our servers.