Stealth ??

It has nothing to do with email. It's a description of a low level networking function. Normally when another computer tries to connect to a closed and firewalled port on your machine it will get a return message telling it that the port is clossed. In stealth mode that message does not get sent to so the other machine does not know if there is a computer at the address or not (and will have to wait for it's internal timeout before deciding that the connection to that port failed). It helps defend against port scans and the like.