We dropped Mimesweeper for Domino and went with securiQ from Group Technologies (
http://www.group-technologies.com). I got burned on the lack of R5 support for the Minesweeper product. It took them over a year to get the product working with R5 code. After we upgrade we had server crashes and smtp hangs. We restored the smtp server to R4 code and waited, again. They tired to keep me as a customer by giving it to us the next year free. I still switched to securiQ.
We have had very good support and few problems with the Group Technologies code. They are still building new version of the product and adding new features.
Hope this helps.
Clay