i guess the question is: are you missing something in the current setup?
postfix + dovecot is a widely used setup that can scale to high user counts.. so i guess it's a good solution.. i use this combo myself, but with mysql for account management instead of ldap (then again, i only have about 3-4 account, it's a private server
) ..
so unless you are looking for some features or functionality which dovecot and postfix can't give you, i see no reason to change to something else if it's working for you.
cheers
Pascal