[NAME, COMPANY AND ADDRESS, ex.
Jill Smith
XYZ Inc.
1234 First Street
Suite 567
Anycity, Anystate  85245]

Dear [NAME, ex. John Smith], 

I just wanted you to know how much I appreciate the extra effort you made to cover for me during my period of absence.

I know how hard you already work, and I am well aware of the additional demands which my workload placed on you. 

I have acknowledged the extraordinary efforts you made, and they will not be forgotten.

I hope to be in a position to reciprocate whenever you need me. Thanks again. 

Sincerely, 

[YOUR NAME, ex. John Jones]