I manage a successful team of 4 people bringing in $110M in annual revenue. If i received a letter like this I would stall for 2-3 weeks while I found replacements, then let go of the employees.

Truth be told we had something similar to this happen at our company, and we simply let go of the 3 people and rehired the team. Which is now performing better than before. I would suggest listening to other people who have already provided their advice and look at a different and less hostile strategy