Zimbra and ISPconfig might be quite possible.
Zimbra has an entire control section based on command line.
So instead of having a postfix script to create an account you add an account to zimbra.
Likely there is something that could be done with LDAP as well.
http://www.zimbra.com/docs/ne/latest...line.20.3.html
For example:
zmprov createAccount
chrisw@domain.com {SSHA}BgR9EwbvVGvxSFChoZs2zoZ cn chris w displayName chris w givenName chris
Or:
zmprov CreateDomain domain.com