Which headers can and should be set when sending out email notifications?

I'm writing an email sender that sends notifications to users about various things they've subscribed too, but I don't have more than 开发者_开发知识库shallow understanding of emails. So I'm curious what email headers can be set, and which ones should be set? Are there non-RFC headers that are useful for email clients, that could be interesting to me?

Any advice is appreciated.

I've just started to learn about email headers as well. I understand that headers starting with "X" are optional and can be your own custom headers used for different purposes.