> There are a lot of mail user agents out there.

Yeah. Also, the usage share distribution is broader and less dominated by the big players than for most other kinds of software.

If you look at web browsers, your usage share is more than half IE, more than half of the rest Firefox, and most of the rest is either Opera or Safari. (If you look at rendering engines, it's even more pronounced, since the next several players after those listed are using the same rendering engines as one of the listed ones.)

Word processing isn't quite that marked, but it's still a very strong showing for the big players (MS Word, OOo, MS Works).

Databases are all about the big four (Oracle, Postgres, MySQL, and MS SQL Server, not necessarily in that order).

With mailreaders, though, nobody has anywhere near 50% market share, and if you throw out the four biggest players (Yahoo, Hotmail, Outlook/WindowsMail, and Google, probably in that order), you've got over 10% of the market left (maybe 20%), and none of the remaining players has anywhere near 25% of that remaining market share. It's really a diverse market. Which, IMO, is a good thing.