Your email needs to do several things.
- Explain what you've been doing. Your boss has lots of employees; they can't remember every detail.
- Let the boss know you've passed and - if appropriate - what grade you got. Demonstrate your worth to the organisation.
- Thank your boss. Perhaps they gave you moral support, or let you take time off to study. Doesn't matter - make your victory your bosses victory. Then they can tell their boss "Yeah, I helped our employee pass certification."
- Thank the company. Yes, it's sucking up a bit, but it never hurts to thank people. Look at Oscar speeches :-)
- Remind your boss about how much more valuable you will be now you are certified. Will it help customers? Does it make the boss's life
- Finally, reciprocate the time and effort spent on you back to the company. You're not going to take your hard won exam and then go running to the competition.
Here's what I'd say - adjust to fit.
As you know, I've spent the last X months working towards ABC Certification. I'm really pleased to let you know that I've passed the exam with a grade of 98%.
I'd like to personally thank you for all the help you've given me over the last few months. I feel really proud to work for a company which is willing to support its employees.
I can't wait to put what I've learned into practice, and look forward to having a bit more free time in the evenings :-)