There is a time to be formal in email and a time to be informal. For example, it is not necessary to include your email signature on every email that you send within your company. However, when reaching out to a new client, your email signature is an expectation for communicating your contact information, company role, and credentials to help reinforce your credibility.
Using a greeting in every email is an expectation, but not necessary in some situations. When cc’ing a group of colleagues on an email, be sure to say something like, “Hi, everyone.” Individual emails should address the recipient by name and title, but follow-up emails can contain a less formal greeting.