Just had an idea to counteract the increase in spamming. At the moment
exim has the option to check RBL domains and based on the result to
reject a message ( /reject ) or add a warning header ( /warn ).
How about providing another option to throttle the connection to a very
low bandwidth ( /throttle ). That way the message is still received, so
you don't reject what might be non UCE, but you avoid your server from
being overrun with UCE (and you slow down the spamming mailserver which
benefits other people also).
In order to prevent exim wasting too many resources to these connections
(ie. they are connected longer), something has to be provided to specify
a maximum number of throttled connections. If the maximum has been
reached, then rbl'ed mailservers get a 4xx error.
