How to differetiate the different version of the software in BUILD_DEPENDS?

[HHChang]

If the software depends on some specific version software, how to =
specify  it?
For example, if the software depends on postfix-2.X, I write the =
Makefile like:
RUN_DEPENDS=3D    postfix:${PORTSDIR}/mail/postfix

But it seems just check the postfix program, if someone had aleady =
installed the mail/postfix-current, the statement won't work well.
Is there any solution to specify it?

Regards,
            HHCHANG