The following is the email sample regarding thanking your boss for the contribution he has done till now in enrichment of your career.

Subject Line(Keep it as formal as possible):

Thank you, YOUR_BOSS_NAME!

Email Body:

Since last few months, I’ve been in YOUR_COMPANY_NAME and saw loads of personalities around me.

Despite the sunny and rainy situation, I was rest assured that my career was in prominent hands and got to do some interesting work under amiable and generous leadership of yours.

The significant help, that you did in driving me out of the mess everytime and guiding me to achieve the heights was remarkably appreciable.

Anew, Thanks a lot!

In above email, there are few noticable things like usage of more formal words(Prominent Hands, Amiable, Generous, Remarkably Appreciable) rather than words which are used informally(Friendly, Guide/Teacher etc).