Microsoft Exchange sold more units than IBM's Lotus Notes in the US last year, but Big Blue maintained its dominant position in Western Europe.
According to preliminary data from IDC, Lotus still dominates in terms of its installed customers' base.
Mark Levitt, analyst at IDC, said: 'This is a big pie and it's growing. Just because Exchange is doing well doesn't mean the others are doing badly. But Lotus does have Exchange breathing down its neck. It will be interesting to see whether it will maintain its lead in 1999.'
IBM added about 13.4 million users to its Notes user base worldwide in 1998, while Exchange gained 11.8 million and Novell Groupwise 4.51 million.
Exchange gained ground in the US but Notes dominated in Western Europe - 3.7 million additional users compared with Exchange's 3 million. Lotus had 29.6 million users at the end of 1998. Exchange now has 21.2 million, followed by Novell's GroupWise at 14.2 million.
Levitt added: 'The value advantage of Lotus has traditionally been that it offers an environment in which custom applications can be developed and run. That is less true of Microsoft.'
The findings also revealed that traditional groupware players continue to prosper, having warded off the challenge posed by intranet-based products.
But Levitt pointed out that potential rivals, such as Netscape with its Suitespot server, appear to have dropped off the radar screen.
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