Consider the effects on your mail server with that much mail being set at
once.  You might need to set some internal LISTSERV variable as to how
big a batch of RCPT TO:s it generates.  Will you want the mail sent via
BCC :or an explicit TO: ?

I would suggest that you consider opt-out including the use of other
e-media including WEB pages and local Usenet.

Also, using confirmation via the OK  mechanism would seem prudent, to
foil spoofers.

Who/How are bounces going to be handled due to "host not found," "mailbox
full," or "no such recipient?"

What are you plans for handling customer REPLY (legit or illegit)?

/Pete