E-mail "Reply" problem

Hi all,

Running an e-mail test.

I am composing an e-mail from the “lead” page. E-mails are being sent.
When I receive an e-mail in my mail client and try to do a Reply, I send replies to myself.
I send e-mails to ABC@mail.com and replies should go to XYZ@mail.com (ABC is a customer, XYZ is myself)

Customer at ABC receives e-mail and hits REPLY button.
New message is composed
From: ABC@mail.com (correct)
Reply-To: XYZ@abc.com (does not seem to have any effect)
To: ABC@mail.com (and this is where e-mails are being sent to, which is back to the customer)

E-mail Client is Thunderbird, but I do not think it matters.

Thanks,
Rudolf