[Exim] acl to reject clients

Hi,

I have been asked by a firm for whom I installed exim for to block
certain email clients from being used when sending email. They want this
for license compliance to force their staff to stop using clients for
which they don't own licenses (ie. outlook, lotus notes etc). I
suggested that they just enforce it, but staff tend to install whatever
they feel like without the management's knowledge (they don't have an
admin, I'm it and only ask me to do work irregularly, so I have no
control either)



Is this possible, and how is this done ? I presume its possible by
inspecting the User-Agent but I don't think outlook express sends this,
and I imagine they would still like their staff to use OE.

thanks