Is there a messaging solution out there (preferably supporting Python) that I can use like a mailbox, e.g. retrieve messages from any given queue without having to subscribe? I suppose message queues could work, but I would have to repeatedly subscribe, grab messages from the queue, then unsubscribe, which does not sound optimal.
Most (if not all) Messaging solutions support two modes of messaging

Publish \ Subscribe -that is, you need to subscribe to get the message.
Queuing - one party sends a message to the queue, the other reads the message from the Queue - no subscription needed, and the message is consumed when it's read.

Actually, standard Queuing is more common then publish subscribe - you have better chances of finding a tool that supports queuing, but not pub\sub, then find a tool that supports pub\sub but not queuing.
You are probably looking for the 2nd mode
Protocol for retrieving and publishing messages (message queues without the pub/sub)