You can use a phrase like, “I hope you are doing well,” or “I hope you had a good weekend” but it’s better if you can personalize it a bit more, like:
I hope things in Tokyo are going well.
I hope you have been enjoying the warm weather we’re having.
I hope you had a smooth trip back from Thailand.
Another way to start an email is to ask a polite question, like:
How are you?
Have you been able to get settled in?
How are things going in Dallas?