Though not actual correspondence, the following is based on actual events.

Dear Realtor A,
I was excited to see a property this weekend, but when I was an hour away, the Realtor called and told me the listing agent had cancelled our showing because it was under contract. What gives?
Sincerely,
Disappointed Potential Buyer

Dear Disappointed,
There are many reasons why this can happen, but usually it is because an offer has been accepted that is cash and has no contingencies. In other words, the seller has accepted the terms and there is no way out of the contract now, no matter what. Even if you made a higher offer. Unless the seller and the buyer in question release each other from the contract, the sale will go forward. To make matters  more disappointing for buyers, the law requires that the For Sale sign will remain in the yard until the property closes. Fortunately, with cash sales, closing can happened quickly, though sometimes, it doesn't. Needless to say, this is confusing for buyers and makes everyone in the industry look bad. Ultimately, though, this convoluted system is in place to protect consumers, right down to the For Sale sign on a property that can no longer be sold. Buying real estate is a complicated transaction, and the best way to prevent fraud from any of the multiple parties involved (buyer, seller, real estate agent, bank, etc.) is to have very distinct steps in place to take everyone from offer to accepted offer to closing, which is when the property is transferred in exchange for money or something else of value. Until then, all the parties are bound by promises they are legally required to keep, and each party must perform the agreed upon steps. Which takes time. Time when the property appears to be available for purchase but really isn't. Backup offers are accepted sometimes, but only if the contract has contingencies, that is, conditions that have to be met by each party in order for the sale to go through, and even then most brokers require a mutual release from a purchase agreement before the property is really free to be sold again. My answer is of course meant to describe the process and should not be construed as legal advice. (Yes, I used to work around attorneys.)
Sincerely,
Realtor A