[Exim] warn users about sending large messages

I don't want to restrict the size of messages my users can send, but I
would like to be able to send them a gentle reminder when outgoing
messages are over a certain size. E.g "your message [subject or sender
name] has been sent, but was over 10Mb in size"


Has anyone done this before? Is it possible?
I use a very simple set of routers as a mail-hub shown below:
---------------
redirect:
driver = redirect
data =
${lookup{$local_part@$domain}lsearch{/usr/exim/exim_redirect_to_list}}

internal:
driver = manualroute
transport = remote_smtp
route_data =
${lookup{$domain}partial-lsearch{/usr/exim/exim_route_to_list}}

external:
driver = manualroute
domains = ! +relay_to_domains
transport = remote_smtp
ignore_target_hosts = 0.0.0.0 : 127.0.0.0/8
route_list = * my-mailscanning.service.com
no_more
-----------------

Thanks in advance,

Jason Meers

________________________________________________________________________
This e-mail has been scanned for all viruses by MessageLabs (outbound).

The information contained in this message is intended for the addressee only and may contain confidential and/or privileged information.

If you are not the addressee, please delete this message and notify the sender. You should not copy or distribute this message or disclose its contents to anyone.

E-mail transmission cannot be guaranteed to be secure or free of errors or viruses. Therefore no reliance may be placed on this message without written confirmation from an authorised representative of its contents.

________________________________________________________________________