It is very likely that "spam mail" can be sent if you can send mail to anyone so long as you know their username, no matter who from. It is also obvious annoying people can be blacklisted. However, each time spam mail is sent the player would want to go see what the mail even is and must then head for a postmoogle or mailbox. When it turns out to be spam that would annoy the player, especially if they are expecting mail from someone when they get the spam. This is why it is best to require recipients be on some sort of list, to avoid all the hassle.