the mail server is built on JAVA, so what i want is some way to call a function in jave to my php page.(i read about something called php/java bridge, but i don't how to use it)
In that case, this should probably be in the Java (or PHP) Forum. However nobody could know, as you didn't give this information. Well, everyone makes mistakes.
The operating system is normally irrelevant, when Java is used (if it is coded apropriately that is).
no limit is defined. but if there, how the solution will be different?
As you probably wrote the server yourself (or at least have the code) it would be easy - you would have to check, if the request to create a new alias is within the limit and only if so allow the creation of a new alias continue.
Then the question comes up: How do the users connect to the mail server at the moment? Is it also webbased or is the alias-project the first webbased part of it?