[exim] Delaying messages for 5 minutes?

I've been working on outbound filtering and trying to come up with a new
set of tricks. Outbound filtering is very different than inbound.

Here's the situation. An ISP has thousands of email users and some have
used week passwords or otherwise been suckered into giving up the
password. The spammer get access and starts sending spam at the rate of
thousands per minute.

I can detect the increase in the speed of sending rather quickly but it
might take say - 5 minutes - to determine if it's a spammer of someone
with a big email list sending legitimate email - and get that
information to my servers. During the 5 minutes the spammer would be
able to send thousands of spams before being shut down.

So - what I'm thinking is that if I have a high rate sender I'd like to
be able to delay delivery of the email for 5 minutes and then either
send the email if that are determined to be sending good email - or
discard the email if they are sending spam.

Just wondering if there is an easy way to tell Exim to hold email for 5
minutes?