When assigning path names to the
forward_path parameter, you can use macros
$user that are expanded
by Postfix to determine the path for the current message. The
parameter specifies a list of characters that should
be allowed when expanding macros. Characters that are not permitted
are replaced by underscores.
forward_expansion_filter = 1234567890!@%-_=+:,./abcdefghijklmnopqrstuvwxyz\ ABCDEFGHIJKLMNOPQRSTUVWXYZ