We appreciate the hard work you have done for (company). We know it hasn't been easy the last (number) months to do double duty as you've covered in the areas where we've been short handed. We do appreciate your willingness to help out where there was a need.
We...
Dear (recipient's name),
We appreciate the hard work you have done for (company). We know it hasn't been easy the last (number) months to do double duty as you've covered in the areas where we've been short handed. We do appreciate your willingness to help out where there was a need.
We have hired additional employees and you will no longer need you to cover in the other areas. You can now concentrate fully on your responsibilities. This should be a big relief for you. Thank you for your patience as we've worked through this time.
Sincerely,
(your name)