If the SMTP server is at a remote location, you need to increase the number
of SMTP workers, eg:

SMTP_FORWARD_1=10*my.smtp.hostname

This is necessary because the SMTP protocol is half-duplex, so if there is a
substantial round-trip time between LISTSERV and SMTP, you need to increase
parallelism to compensate.

But assuming the SMTP server is on the same LAN, it sounds like it is
purposefully limiting the rate at which it accepts messages from LISTSERV,
perhaps as part of a generic anti-spam package or something like that. You
then have to look at the SMTP server logs and configuration to see what can
be done.

  Eric