That's very bizarre. The forum software calls ns_sendmail once for each e-mail address, and weeds out duplicates beforehand.
ns_sendmail opens a socket, builds an SMTP message, ships it off to the addressees, and closes the socket. The forum software does an individual ns_sendmail for each person's alert message, which is why the names pop ... up ... one ... at ... a ... time ... slowly on the post-confirmation page shown to the user.
I've never had anything like this happen on my server - I use my ISP's SMTP server. I've never received bombardment like this from any ACS forum I've asked for alerts from, and that's quite a few forums both OpenACS and ACS Classic.
I vote for your e-mail server configuration unless others confirm they've had the same problems.
