Dear Mr./Ms. Bossname,
I’m reaching out to you because my immediate supervisor, Marvin Middlemanager, is on vacation.
An opportunity has come up, and I must tender my resignation. I wanted to let the company know as soon as possible. I’m able to give the standards two weeks’ notice.
I regret the timing of my resignation, but I believe that we have things well in hand here at the department, and I am already preparing materials that will ease my coworkers in the transition.
I am copying my boss on this letter so that he can review it upon his return. Thank you for your assistance in this matter.