There are some add-ons which do this; I found one after asking here - search for POP3 and you will find the earlier thread. There are also quite a few other utilities which do this on a standalone basis - see www.vpop3.co.uk for one example you can set up on a spare, small PC and have firing SMTP at your existing Domino server.
Steve Cassidy
Networks contributing Editor, PC Pro Magaine
PPA Business Columnist of the Year, 2004