Don, many thanks for your prompt reply!
I'm running this at the end of a DSL -- 784-1.5Mdown/128up. So maybe that's it.
Glad to know that ns_sendmail *does* use the default MTA. But if the problem is my uplink bandwidth, does qmail's queueing mechanism allow a better buffer against this limitation than does exim? And is the OpenACS site running on a big pipe in addition to using qmail?
Thanks again!