Here are some email marketing ideas I can think of that may be effective:
1. Monthly themes/newsletters with health tips, seasonal advice and clinic updates.
2. Focus on prevention, you can also do themes, such as breast cancer awareness month in October you can promote mammograms and explain why it is important to get mammograms and be preventative. Another idea is heart screenings in February for valentines day.
3. Create a "Did you know" section where you give people tips about common conditions, how to spot early signs of illnesses/sicknesses/cancers etc.
4. Myth vs. Fact, create some myths and facts and see if people can spot the myth and/or spot the facts such as only having to wear sunscreen in the summer vs. the truth which is we should be wearing sunscreen all year round.
5. You can also have a FAQ section where you answer popular questions
6. Create a Meet The Team page where people can get to know the team
Those are the ones I can think of right now that I feel would have a good response and help market the clinic. Make it interesting so clients want to stop and read the newsletter, if you make it too confusing, long or use too much clinical jargon people may not be as likely to read the emails.