A listserv address is a mailing list that multiple recipients subscribe to. An email sent to that one list -- like info [at] company [dot] com or communications [at] example [dot] com -- reaches many.
We don't allow sending to a listserv for a variety of reasons. First, there are some serious permissions issues that need to be considered. While these people have opted to receive those listserv emails, but that's not exactly the same as opting to receive your emails directly, and that's a gray area.
The other problem is that anyone can click to opt out of the mailing, and that will remove the entire list. If the list is posted online, that means that anyone can click that link, including search bots, which will guarantee that this address will be continually opted out.
The best bet is to either deliver the information to the listserv in another manner (a direct email from you, for instance), or give folks at the listserv a chance to sign up for your newsletter by pointing them to your signup form. We recommend both, at least until you have your signups squared away. In order to opt an email address back into your audience, we have to have permission from the recipient of that address, and since no one can legally speak for the entire list, we won't opt in that address.