When you send mail in this system, your mail will be encrypted by your public key, and you need private key to unlock the mail for reading.
FYI, this is wrong. It has to be encrypted by the reciever's public key, not your public key.
It has to be encrypted by your reciever's public key to ensure only they can decrypt it, and it has to be signed by your private key so they know it's coming from you.