A related problem is that ns_sendmail (in aolserver/modules/tcl/sendmail.tcl) discards everything but the <email@example.com> part of an e-mail address.
The notifications package is configured to send out messages from
[address_domain] mailer <[reply_address_prefix]@[address_domain]. In the case of OpenACS these messages would show up as coming from 'OpenACS mailer' if it wasn't for ns_sendmail.
My vote goes to 'fixing' ns_sendmail, adding a Reply-To header to notifications and replacing the from address with the name and e-mail of the poster (for forums).