[Exim] spam delay trick and smtp_accept_count patch.

Dear all,

An anti-spam trick was suggested to me a few weeks ago by a colleague in
our university. The idea is that we add longish delays into the smtp
conversation if we suspect that the sender is a spammer. i.e. if the
sender is on an RBL list we add an acl like:

accept  dnslists = rbl-plus.mail-abuse.ja.net
                  delay = 1m

My colleague has reported that this trick is successful at reducing the
amount of spam he sees being delivered into his department.

However, I am concerned that my process tables will fill up with RBL'ed
hosts connecting to my server and filling up my (smtp_accept_max)
connections. So, I'd like to defer RBL'ed hosts at a lower limit (to get
rid of their connections). A one line patch makes the number of
connections available in the ACLs. Note patch is against exim-4.14 (or
maybe 4-20):

--- a/src/expand.c      2003-03-11 12:20:20.000000000 +0000
+++ b/src/expand.c      2004-01-16 23:01:27.000000000 +0000
@@ -292,6 +292,7 @@
   { "sender_ident",        vtype_stringptr,   &sender_ident },
   { "sender_rcvhost",      vtype_stringptr,   &sender_rcvhost },
   { "smtp_command_argument", vtype_stringptr, &smtp_command_argument },
+  { "smtp_accept_count",   vtype_int,         &smtp_accept_count },
   { "sn0",                 vtype_filter_int,  &filter_sn[0] },
   { "sn1",                 vtype_filter_int,  &filter_sn[1] },
   { "sn2",                 vtype_filter_int,  &filter_sn[2] },

Then use a condition in the ACLs to reserver some connections for non
RBLed hosts:

  defer   message   = Too many connections for RBL listed host. Try
later.
          condition = ${if and { gt {$smtp_accept_count}{50} }
                               { eq {$acl_c0}{spam} }
                               {yes}{no} }

Note that in our configure file $acl_c0 is set to spam if the submitter
is listed on our favourite RBLs). We have smtp_accept_max=90 so this
setup reserves a decent number of connections for non-RBLed hosts.

Also note that smtp_accept_reserve is used to list our organisations
internal hosts... so I don't want to use it for this reason.

Two questions:

1. What to people think about this delay trick?

2. What are the chances of getting this patch included in a future exim
release?

Cheers,
Martin.

--
-- Dr MDT Evans, Computing Services, Queen Mary, University of London