People who message you on Facebook are usually those you have accepted as 'friends', but from time to time a stranger will send you a message.

When that happens, Facebook uses a 'message request' feature, that sends you a notification if someone you are not friends with tries to contact you.

But it turns out this isn't always the case. Facebook will only notify you if it 'thinks' you might know the person. Anything else is filtered out as spam.

"But what if those 'spam' messages aren't actually spam?" I hear you cry.

Fear not. It turns out there is a hidden folder in Facebook Messenger where all these sad, neglected messages end up.

Here's how to find it:

Go into the Messenger app
Tap on the Settings icon
Tap on "People"
Tap "Message Requests"
Scroll to the bottom and tap "See filtered requests"
For the most part, the messages that appear in this hidden inbox are exactly what you'd expect - spam and messages from people you've never ever heard of (or want to hear from).

However, as is the case of one man who discovered a message from the partner of his estranged dad urging him to contact the hospital as his father was dying - three years after he died, some messages are very important.

Facebook ditched the 'other' inbox last year in favour of message requests, which allow you to accept or ignore new requests without the requester knowing whether you've read their message.

Facebook said at the time that it would "ruthlessly combat" spam messages, to stop them cropping up as requests in your inbox.