I know how frustrated you must feel right now. It's difficult to lose a big sale, especially when everything looked so promising. You did your best to meet the needs of the client. Unfortunately, they chose to go with another company. Don't worry too much about it. Of...
Dear (recipient's name),
I know how frustrated you must feel right now. It's difficult to lose a big sale, especially when everything looked so promising. You did your best to meet the needs of the client. Unfortunately, they chose to go with another company. Don't worry too much about it. Of course, the sale would have been nice, but there will be other sales. We will press forward and continue to do what we do best---sale amazing products.
Sincerely,
(your name)