The title and content of every message is created by the person that sent the message. Perhaps this is who you should ask about the meaning.
Unless it has already been fixed, there is a current bug which prevents some people from sending mail in the first attempt. Since the message disappears, the sender might be unsure of whether or not they sent the intended message. The workaround to this is to copy the message (Ctrl + C) and if the doesn't appear in the list, try again (paste it Ctrl + V).
Trading accounts is not against the rules. Having more than one account on a world or having more than one person log into an account is. So sending someone your password is not a banable offence. Foolish yes, but not banable.
If someone is sending you login details to another account and asking you to break the rule, leave the message in your inbox and send a support ticket. The 'report message' button sends the message to the wrong area of support and will generally only be assessed for language or other illegal content.