Not sure if we are "the biggest", but we are certainly up there. It's crashed IIS for us since day 1 of testing MAPI back when the beta launched. It's been a roller coaster of devs working late, devs working on weekends, process dumps, tuning, running traces, Fiddler and on and on for the past almost 4 months. SmarterMail is finally in a good place for the most part, just have this issue with MAPI crashing IIS. It happens with just 1 user connected to MAPI. IIS will run for months without a twitch, but the moment MAPI starts being used it crashes within hours and will continue to crash until the Outlook install is shutdown.
Larry has been working to figure it out, but so far, nothing. Right now, our next option is to fully reinstall the server with a fresh Windows install and a fresh IIS installation. Just to see if it is something weird with IIS.