If you are looking to exchange greetings with colleagues or friends over email, here are some suggestions:
Just writing "Happy Thanksgiving" to different colleagues can sound quite awkward. So, try this:
"Hi! Happy Thanksgiving! Hope you and your family have a great holiday."
It's simple but still could work, considering you want to keep it professional.
You can also go for:
"Wishing you a Happy Thanksgiving from your friends at {Company Name}. Have a safe and happy Thanksgiving with your family and friends."