郁闷啊，自动分拣退信的功能失败了
2005-09-05倒腾 postfix，想将退信自动分拣到不同的目录，结果失败。查找恁久的manual，就是找不到原因。后来日志里面提到了：
warning: regexp map /etc/postfix/maps/map_virtual_mailbox_maps, line 1: regular expression substitution is not allowed: skipping this rule
再去 google，居然找到了，可是奇怪的是google 出来的 virtual(8) 跟我看到在postfix官方网站上看到的 virtual(8) 居然不一样。
我郁闷啊
virtual_mailbox_maps
Recipients are looked up in these maps to determine the path to
their mailbox or maildir. If the returned path ends in a slash
(“/”), maildir-style delivery is carried out, otherwise the path
is assumed to specify a UNIX-style mailbox file.
While searching a lookup table, an address extension
(user+foo@domain.tld) is ignored.
In a lookup table, specify a left-hand side of @domain.tld to
match any user in the specified domain that does not have a spe-
cific user@domain.tld entry.
Note that virtual_mailbox_base is unconditionally prepended to
this path.
For security reasons, regular expression maps are allowed but
regular expression substitution of $1 etc. is disallowed,
because that would open a security hole.
For security reasons, proxied table lookup is not allowed,
because that would open a security hole.