Acredito que isso pode ser corrigido com mod_setenv :
$HTTP["url"] =~ "^/webmail" {
# add host header
setenv.add-request-header ( "Host" => "example2.org" )
proxy.server = ("/webmail/" => (
# this entry should link to example2.org
("host" => "1.2.3.4", "port" => 80)
))
}