Libel is a tricky business. Has your reputation been lowered in the minds of the ordinary or right thinking man?. This is the test. Who else, apart from your employer has knowledge of this letter??? also,If your employers do not believe this allegation then you have not suffered.
Plus, if you do take an action, the costs will be phenomenal.
Bruce Grobbelar sued the sun for libel in the 80's or 90's over the match fixing allegations. He won his case... and was awarded �1.!!!