Facebook founder and CEO Mark Zuckerberg. Justin Sullivan/Getty Images Facebook has a hidden message folder where it filters messages it thinks you don't want to see — sometimes with serious and unintended consequences.

Users are complaining about not seeing messages from since-deceased family members, people reaching out to help them, and even people contacting them to let them know their friends have died.

Back in October 2015, the social network revamped its messaging system.

Previously, you had your "Inbox," and an "Other" folder where messages from people who aren't friends or friends-of-friends would end up. But it replaced this second folder with "Message Requests" — which let people you're not connected with ask to be able to talk.

But what very few people realise is there is now also a third category of messages: Filtered Message Requests. These filtered messages are what Facebook thinks is spam, and so doesn't even notify you about. They're also not easy to find.

There has been a spate of articles over the last few days explaining how to access them. Judging by the reaction on Twitter, almost no one knew it was there.

—Max von Essen (@MaxizPad) April 6, 2016

A lot of the time, the feature appears to be fulfilling its purpose pretty well — protecting people from strangers' spam they've got no interest in viewing.

The message Daniel Emery received informing him his friend had died — and which he only discovered two months later. Daniel Emery

But there have also been cases of it hiding far more important messages, meaning their intended recipients only see them when it's far too late.

Former BBC technology journalist Daniel Emery contacted Business Insider after reading our story and checking the "filtered" folder out of curiosity.

He told us that back in February, the partner of an old college friend messaged him to tell him that his friend had died — but the message was filtered, so he only saw it today, two months later.

Emery said: "He was a friend of mine from when I was a student in Aberdeen. Used to be a good mate, obviously lost touch a few years ago, as you do."

He had thought about reaching out, but never got round to it — and then saw the message letting him know his friend had "popped his clogs."

"So its his partner — I don't quite know who she is — picking up the pieces ... it's all a bit strange really."

Emery's not the only one to have experienced something like this.

Matt Spicer, from Bristol, England, said that his wife started crying after discovering the "filtered" inbox — because it contained a message sent by her cousin, who died before it was ever read.

And Brittany Knight wrote on Twitter that she lost her passport and missed the message from the person trying to return it to her due to the filter.

There are many more cases like this. Some are recent, while other messages appear to have been sent before the Filtered Message Requests inbox was introduced, and were then automatically moved to it once it was launched — all without ever being read.

—Resting Face Palm (@bm0nies) October 30, 2015

The situation is made doubly frustrating by the fact that one of the big reasons for the change to Message Requests (and Filtered Message Requests) is to prevent things like this happening.

Tony Leach, a product manager at Facebook, told TechCrunch back when Message Requests launched: "We’ve heard so many stories like estranged parents trying to get back in touch, or you lost your wallet and someone trying to get in touch with you ... That’s why we want to replace that with a system that makes it a lot easier to catch the messages that you want to see."

Clearly, this hasn't been a complete success.

But what can Facebook do about this? It's a tricky one. If it errs on the side of caution, and filters more, then important messages will continue to fall through the cracks. But if it relaxes its filters, then more spam will likely get through too — and people will complain. (Facebook did not immediately respond to a request for comment.)

Daniel Emery, who missed the message telling him of his friend's death, would like Facebook to modify its messaging feature to add a more straightforward spam folder.

"Gmail and Google, for all its flaws, has a pretty sensible system," he told Business Insider. "There is a folder for spam, you can look into it if you want, you can pull things out. It's not hidden from you, and I think the issue with this is it's kind of a bit too much 'nanny state.'"

He added: "Facebook is trying to be overprotective. Overprotective comes at a cost, and things like this happening are exactly that ... hiding it is never a good thing."

If you want to check your Filtered Message Requests in the mobile app, load up Settings, then click on "People," then click on "Message Requests," then click on "See filtered requests." If you're on the desktop version of Facebook, click on the drop-down Messages tab, then on "Message Requests," then press "See filtered requests." For more detailed instructions, read this »