They only reason for using a single mysql instance in largeinstallations in a single namespace for shared mailboxes. If that is nota requirement you may consider spreading the load.
For large installation, I would put my money on PostgreSQL, MySQL base is based on delivering fast transaction but not on data integrity.
Also lately seen on slashdot: http://blog.develix.com/frog/user/cliff/article/2006-06-04/9 /Marc
