"In some UNIX systems, the link(symlink, newpath) operation has changed over time: instead of recursively following the symlink and creating a hardlink to the file thus found, it creates a hardlink to the symlink itself. This behavior disagrees with, for example, the POSIX.1-2001 and X/Open XPG4v2 standards, and is the default on current Solaris, IRIX and Linux systems. On systems with this non-standard behavior, Postfix may be vulnerable depending on how it is configured."
IEEE Std 1003.1-2001: http://www.opengroup.org/onlinepubs/009695399/functions/link.html