It is true that the majority of Lotus customers implement Notes/Domino to do "more than mail". But there is still a significant minority that use these products just as an enterprise class mail system.
Both Gartner Dataquest and IDC say that Lotus is #1 in this category (Gartner says IBM #1 in messaging with 44%, 8% ahead of the nearest competitor).
http://www.lotus.com/compare has all of our public information. There's also my "boss loves Microsoft, where does that leave Lotus?" presentation,
ftp://ftp.lotus.com/pub/lotusweb/press/Boss_loves_microsoft.pdf (that link might be broken, let me know if it doesn't work).
If you have specific questions, I'm happy to help... ed_brill@us.ibm.com . Also some content on www.lotus.com/weblog
--Ed / IBM Lotus Senior Marketing nitwit