You can sign any e-mail by attaching the public key.
If you want to encrypt an e-mail, you need an e-mail from the recipient that he signed with his public key. The best way to store this public key is with the contact details of the sender of this e-mail. With this saved public key of the recipient, you can now encrypt an e-mail to this contact. This encrypted e-mail is automatically decrypted with the recipient’s public key.