Ok - so the solution was short lived... problem reappeared on both my SMTP servers. Here's what I've done since that has (so far - it's been almost a week now) fixed the issue on both my boxes...
http://www-10.lotus.com/ldd/nd6forum.nsf/55c38d716d632d9b8525689b005ba1c0/669898e5459b536c85256dbe00734113?OpenDocument