As long as you keep it internal-only, get a reliable server that doesn't do any anti-spam shit (AKA, nothing from MS or any other large company), doesn't use some unreliable proprietary database (again no MS), and have a reliable journaled disk, your only important failure mode will be losing disks. That's an easy failure mode to deal with.
You also need to add misbehaving clients on your list of things not to use with a message-queue, just opening the queue's folder on Outlook to inspect it may be enough to completely corrupt it (Outlook will happily reorder, even rewrite, header fields of incoming messages, replicate messages to infinity, or who knows what else).
I'd be worried to have a message queue that is compatible with such a cursed ecosystem and transport protocol.