Can you elaborate how Exchange was unsuitable and which was your proposed solution instead? I hope Notes wasn't the answer... :-)
Once the companies' execs discover what else they could do with their Outlook software instead of the basic IO (SMTP+IMAP/POP) there's no looking back. Scheduling meetings, accessing other people's resources and such is nice and integrates well into Outlook and most mobile phones.
If you really are after just IMAP/SMTP then a Linux based solution is probably the best answer.