Nice plugin. I wonder if it would be possible to configure this to be multi-user. That is, which users who come to the blog as members could use the WP Responder to make unique newsletters and send them under their own address?
I see that the "Allow Multiple Accounts" plugin allows for more email addresses than one, but actually I would just think it would be a matter of a simple entry box for return e-mail in the "from field" of the form/newsletter.
Maybe the outgoing form management could be managed with some other plugin like "Role Manager" or "Capability Manager".
I could see this being an excellent enhancement to an already quality plugin.