There’s a great post on this subject at Are greetings and salutations redundant in an mail?
Mail is a quick, efficient way of communicating. It has a problem. People often come across as rude. Some people are rude. There’s no getting away from that. Others sound rude even when they don’t mean to.
To get around this, I start mail with ‘Hi’. If I know the person’s name I use it. This is respectful, but informal and short. It doesn’t carry any baggage.
Any other word here seems wrong. Mails that start with ‘good morning‘ or ‘good evening’ are polite, but make potentially wrong assumptions about when mail is read. ‘Dear Mr…’ sounds like something from Jane Austin.
‘Hello’ is acceptable, although it can sound twee. ‘Hey’ works if you know the other person well and want to sound jaunty.
I sometimes remember to write ‘Kia Ora’ when mailing another New Zealander. If the phrase offends people, then that’s an added bonus. It can get an odd reception from overseas.
There’s no need to sign-off with anything at the end of a message. I have a signature at the bottom of my mail. That’s to let people know how to get in touch. Nothing else.