Email forwarding a security risk?

I have several mail accounts. One is on an exchange server
and usually accessed with Outlook, which works poorly and
is not accessible from outside a certain protected network where it is used.
I therefore asked for my email to this exchange account to
be automatically forwarded to my mail account on Google.
The exchange server administrator agreed, but now he is
whining that this is a security risk. How on Earth can
simply forwarding mail messages be a security risk??