Had an OpenACS related mail bombardment almost a year ago, but I am pretty sure we committed a fix.
It had something to do with the contents of a forums notification not being quoted correctly and a period on a single line caused the bombardment (it just happened to be in the middle of the message, which was interpreted as “end of message” causing the first part of the notification to be sent out before OpenACS was finished writing it to the filesystem IIRC).
Although it does not sound like your problem, check if the message has a single period on a line.
Ben, I hope you are not torturing this user as you figure this out. If you do not know how to get to the root of the problem, figure out a way to use your thumb (like http://en.wikipedia.org/wiki/Hansje_Brinker) in the meantime: e.g. change that users email address in OpenACS to a temporary gmail account (this is assuming the problem is caused by OpenACS) so it acts as a big bucket.
