[Exim] MySQL lookup acl language

Hiya!

I need to configure exim, using smtp_acl_connect, to selectively prohibit
incoming connections right at the start - without even displaying an SMTP
banner. (I've tested by placing a 'deny' in the acl - and it works just
as prescribed - that way - and denies all connections.)

At this point, I have a MySQL database called 'exim4' with a table called
'blocks' with a column called "address" which I need to check for, and
deny if a match is present.

I need a hand designing the ACL to do this..

The query I need to execute is this:

:/
select address from blocks where address = '$sender_host_address'
:/

I need this acl to selectively deny connections based upon the fact that
the $sender_host_address is found in the database. If it does NOT find
any entry for this, I want the acl to 'accept'.

Furthermore, I'd like the acl, if possible, to default to 'accept' if the
database doesn't respond - to ensure that a database failure doesn't take
the mail system down. (I can get around this if it is not possible..)

Could one of you kind folks help me out with this? I am not too good at
this acl language, and could definitely use a hand. :/

Thanks much in advance for any help you could provide.

Chad Schwartz
CornerNet System Administration