Maillist sign-up: create compound 'from' address

Hi,

I want to have a sign-up form to automatically add users to a mailing list. The form will just ask them for their email address. For the address to be added to the mailing list, the form needs to send an email to an address of the following form:

listname-subscribe-john=gmail.com@[…]

where ‘[email protected]’ is the subscriber’s address. As you can see, I need to replace the @ with an =, then form an email address and send a blank email to it.

Jetpack forms doesn’t have this functionality, and I can’t get it to work with Contact Form 7. Does anyone have any ideas for code / plugins that I could use to make this work?

Thanks!