Are you sure of that? There have been alternatives to Microsoft Office for decades. Yet most businesses use and pay for Microsoft Office, even though their employees most likely don't need anything that doesn't exist in those alternatives.
Why would it be different with email?
I don't think you understand what I was writing about - none of that is MS Office. Thats another topic, but without this (and say some sort of domain propagation rules) bigger corporations will never move out of MS.
Nobody got fired for buying ~~IBM~~ Microsoft. People trust Mozilla though, they've built their brand on not sucking as bad as M$ and Google