Weird extra member in facebook group chat?

[Annie]

I have a facebook group chat with me and 4 friends in it, we've had it for years and it must have over 300,000 messages in it. When i see the messages on my phone and look at the members section of the chat i see my 4 friends names and then this other extra weird name which I can not click on. The name/email address is: mailer-daemon@mx.facebook.com
It is not a link like my friends names and can not be clicked on.
When I log in on my phone I do not see this, it is just my friends in the chat.
What is it? because I'm scared.
This email address is appearing as one of the members in the group chat along with my friends, I have not received any actual messages from it.
There used to be more members in the group chat that left months ago, none of whom have blocked me. This extra 'member' has only appeared recently.

[Nick]

have you ever sent an email to a recipient but you spelt their name wrong? this returns an error message from the user "mailer-daemon" to tell you the message did not send to the person you wanted to. my best guess is that facebook has the same thing to tell you whether or not maybe the person you're trying to message has delted their accout or simply never had one. there's no cause for concern here, nothing is going to happen and nobody is certainly spying on your messages.

[Caliban]

Facebook sends this message out when someone has blocked you from sending messages to them.
It also appears when that 'someone' has deleted their account. The problem is that there is no way of finding out the real reason.

[Daniel]

Facebook Sends that Message When Someone Has Blocked you or Has Deleted There Account

[Monty M]

Yes

[Angeleyes]

Facebook sends out this message when someone has either blocked you or has deleted their account.