This is the sender of the email address.
a null reference if no Sender header field was present in the message
The RFC states that this field can be used if a secretary is sending an email for someone she is working for. The email here will then be the secretary's email, and the Reply-To field would hold the address of the person she works for.
RFC states that if the Sender is the same as the From field, sender should not be included in the message.