What does the sender need in order to digitally sign an email?

What does the sender need in order to digitally sign an email?

• The public key of the recipient
• The public key of the sender
• The private key of the recipient
• The private key of the sender 

EXPLANATION

A digital signature is created by hashing a message and encrypting the hash with the sender's private key. The recipient can then decrypt the hash with the sender's public key. The recipient's keys are not used for a digital signature.

The sender's public key is NOT required to sign the message.
The sender's public key (in the possession of the recipient) is used verify signature.
The recipient's public key is meaningless in this scenario.
The recipient's private key is meaningless in this scenario.  

A general overview of this process can be found here https://www.docusign.com/how-it-works/electronic-signature/digital-signature/digital-signature-faq

SOURCE

https://en.wikipedia.org/wiki/Public-key_cryptography#Digital_signatures