[Exim] Problem with netmasks

I use exim 3.12 (8 Jun 2001) and had an interesting problem with
netmasks recently; to prevent relaying, I had configured a netmask of:

194.203.158.80/4

which is the subnet that we have connected to our outside network

However, it was letting some domains use our site as a relay; for
example

195.224.76.132

I have no idea why this is the case; perhaps it was treating '4' as the
start of '48'? It was disallowing some IP addresses (such as
129.215.153.197).

Is this user error? The documentation doesn't say that it has to be any
multiple of '8'.

Alex.