Thank you very much for the proposal you submitted January 21st, 20XX in response to our request for [describe]. Of course, we are very much aware of the great deal of time and effort that went into your bid. However, we regret to inform you that we have awarded the contract to another firm. We based our decision on [first reason for rejecting bid] and [second reason for rejecting bid or choosing a competitor] .
Nevertheless, we will certainly be inviting your firm to bid on future projects.
Once again, thank you for your proposal.