Formal email:
Dear Mr./Mrs. Gomez/Hokke,
I hope you are doing well.
I am writing you this email to inform you that I am not able to attend your lessons. Unfortunately, ......
I look forward to hearing from you / Let me know if you have any quesitons
Best regards, / Sincerely / Faithfully
Jack
Informal e-mail
Hi Jack,
How are you doing? I am coming to the Netherlands during the Christmas break. Are you free? It will be great to catch up.
Let me know if you are available!
Have a lovely day!
Bye/ Cheers.