How to start an official email to a professor I want to send an email to a professor to inform him that I want to accept the offer of PhD admission and Research assistantship. But I do not know how to start email. Is the following good? if not, would you please recommend better phrase.


  Dear Professor,
  
  I hope you are keeping well ....
Congratulations on finding a PhD position :) When sending an acceptance e-mail to a professor, I would say the "general" rules of communicating with professors via e-mail apply.


Be polite, and extend thanks when appropriate, but don't overdo the social pleasantries.

The polite part basically holds for any e-mail and live correspondence.

I would say that "I hope you are keeping well" would be a bit unnecessary.
Be short and to the point. Respect the professors time.

Professors receive many e-mails a day, and are usually very busy. Saying what you want to say clearly not only shows respect for their time, but also makes it more likely that everything written will be carefully read.
Do at least a basic spellcheck.

Especially in the beginning of a communication. It doesn't cost much time and effort, but could leave a bad impression.
Do not attach big files unless explicitly asked for.

Anything larger than a few MB should not be sent unless asked for (e.g. sending all your credentials / application papers when first contacting somebody is bad), and especially not to somebody who you are not collaborating with at the moment.


Beyond this, there is not much else. I already feel like it's hard to call any of these rules, they're just guides based on common sense. Just say what you need to say. I put and extended the comment by @user11259 here as an example:


  Hello Dr./Prof. Brain,
  
  I gratefully accept your offer of admission and research assistantship, and I look forward to working with you.
  
  (Please let me know about the next steps in the admission process I need to take.)
  
  Sincerely, 
  
  user11259