With the advent of the administrative web interface (as opposed to the
browse-only web interface, which didn't need to issue LISTSERV commands),
you're expected to take care of outgoing mail in a separate process.
You can do this with the old ASYNCH_SMTP feature but I suggest
using SMTP workers instead as this is more flexible. Just add something
like:

SMTP_FORWARD_1="2*my.smtp.server.com"
export SMTP_FORWARD_1

This creates 2 extra processes whose job is to clean up the outgoing
spool directory. LISTSERV is then free to process commands as they
arrive. Try it, it is night and day.

  Eric