b) When you receive someone else's mail.
c) When you receive mail or an e-mail saying you have lost a family member, your house burnt down, you lost your job etc.
b) Hey, Mr. Nicolas! I got you mail by mistake! I didn't know you were a hooker!
c) I got fail mail from my boss. He fired me for being late yesterday.
in a different context "What's wrong with him?" "He just recieved some failmail" or "some failmail just arrived in the post"
or can be used as an insult; "Thats a terrible idea, check your inbox for some failmail you bad"
Fail Mail: When a user's email gets stuck in their Outbox and refuses to be sent, for any reason.