Email is inherently informal. This does not mean there are no formal email messages and no people to be addressed formally in email. All people should and want to be addressed respectfully.
But, if unsure, you can often get away with dispensing of the difficult problem which title or name to use in your salutation altogether.
Do Away with Titles and Names in Emails
- start your message right away (without any greeting line), or
- you use a short informal greeting like "Hello!" or "Hi!" or
- maybe begin with a more conservative "Dear Ms. Lastname,".
It's Great to be Able to Say "Call Me Cynthia!"
Of course, with all default joviality and instant familiarity that comes with email, do not forget it feels great if somebody invites you to call them by their first name.
It is truly an honor and a pleasure both to give and to receive. When should you opt for formal and reserve the informal for later? It depends. In general and unless circumstances dictate otherwise, call others how you prefer to be called.
Don't Beat Yourself up
Do not ever beat yourself up over the right way to address any person or the proper words to start any email. Remember: whatever you say or do, it can be dealt with constructively and creatively later. You're doing the best you know, and it will have been good enough.