What is the right amount of exclamation points in a formal email? Where are they appropriate if you want to come off as friendly but professional?

I would say that normally formal communications, whether an email or on paper, would exclude . Exclamation points are intended to convey surprise, alarm, excitement or occasionally humour, none of which are typically required in a formal email.

To convey an friendly but professional attitude, use shorter sentences and simple vocabulary, but avoid slang.