Actually, it's not that I need a replacement for qmail, I already found one: Postfix. I need a replacement for the rest of the email stack, that works well with Postfix. My current setup constists of qmail, vpopmail, qmailadmin, and few others.
The stack should be able to handle a lot of email for a few hundreds of virtual domains. Email accounts for the virtual domains have to be managed through the web (adding new accounts, forwards, mailing lists, etc), but adding new virtual domains can be done in CLI. Of course, there should be a POP and IMAP server, and some antispam protection.
So, what software do you guys use in the wild together with Postfix?