Yes, it's actually somewhat of a decent (if cursed) message queue for many usecases too. Not to mention the debuggability (you already know how to use an email client).
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.
I love email, and to me it is a very important part of our infrastructure, but it is quite cursed as an arbitrary transport. The semantics are very complex, and any transported data will generally need to be encoded as base64, which is very inefficient.
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.
