When a coworker is leaving, it is appropriate to send a message to express your appreciation for their work and to wish them well in their future endeavors.
- We're going to miss you around here.
- Best of luck to you in your future endeavors.
- We wish you all the best in your new venture.
- You will be sorely missed.
- We hate to see you go.
- You will be missed by all of us.
- Best wishes for your future.
- We will always remember you.
- We hope to see you again soon.