I'm not sure if the JET (Exchange/blue) engine is better than SQL. Or if it tackles a different problem, but from my basic understanding of Exchange, I'm not sure why SQL Server doesn't work. There are plenty of mail add-ons, telco type applications, etc. that use SQL Server or Oracle (or DB2) and perform great. So why doesn't the Exchange mail store fit for SQL Server?

It's a knowledge issue. If there are things that JET does better, list a few of them and let the SQL team work on them.