qmail can do that in its sleep, and scales to very large numbers of accounts just fine.
...but it also can be a hassle to set up and maintain thanks to djb's repackaging restrictions. we used it for awhile but switched to alternatives like courier and postfix, which are included as packages in our linux distros.
Anything M$ related *can* be a time bomb.
or, worse yet, silent corruption that you never even know about. during our move from exchange 5.5 to courier, we discovered silent, unrecoverable corruption in the message database that would have gone unnoticed had we not exported the entire mess to another system.
