This is super-nitpicky, but I think "Reply to Author" would be more appropriate here*. "Sender" technically refers to who physically sent the message, not who wrote it. From RFC 2822:

> The "Sender:" field specifies the mailbox of the agent responsible for the actual transmission of the message.  For example, if a secretary were to send a message for another person, the mailbox of the secretary would appear in the "Sender:" field and the mailbox of the actual author would appear in the "From:" field.

A good example of this happening in practice is an email discussion group, which will set the "Sender:" field of members' messages to be the list's email address.

* Sadly, Thunderbird doesn't do this.