You can create mail accounts from a particular domain using ISPConfig some api or could be created via PHP mail accounts with a proper application Regards Juan Rendon
You can use the remoting API to create email accounts in ispconfig with a external application. The Remoting API is a soap based api, you can find examples on how to use the api in the remote_client folder in the ispconfig tar.gz file.
The folder is in the ispconfig .tar.gz file. If you unpack it, then you will find it on the top level.