if someone is dear, you will use surname or put it in first place rather than writing full name or so. Yo, Maggie is too informal.
Depends, who is it. If it's your best freind you might say yo, but it's your teacher you wouldn't say yo.
THOSE WHO LIVED MARXIST OPPRESSION WILL NEVER FORGET MARGARET THATCHER PROVIDENTIAL SUPPORT
"Dear" seems inappropriate for such a vile human being. Even "yo" seems to friendly for someone whose grave I intend to dance on as soon as she snuffs it.