Intentiaonally queue mail during Exchange maintenance window?

Hello:

I have two FortiMail devices in a config-only HA cluster, running 5.2.6 - they sit between our internal MS Exchange server and the outside world.

We have an upcoming maintenance window for Exchange, and I want to make sure that we don't lose any mail.  I know that FortiMail will queue inbound mail if the Exchange server isn't available, but I'd like to have control of that process, so that the Exchange server doesn't recieve mail during the maintenance window at all, rather than just when it is actively being updated.

Is it possible to manually queue mail destined for our domain, or will I need to do this through my firewall (i.e., disallow SMTP traffic in to the Exchange server, which will force the FortiMail to queue inbound mail)?

Thanks,

Thom