I would like to have said it is common courtesy, but we’re in an age where we shoot email back and forth without our names attached beyond a built in ‘signature’ and where everything is focused forward. The simple act of saying ‘Thank you’ has become a rare thing when we don’t like what we have received.
It is easy to remember this when they took the time to carefully read and make editorial notes before sending it back. It is simple to remember to show gratitude when they send a hand-written letter with a few tips and useful bits of advice. It isn’t so obvious a thing when they are offering limited if any feedback aside from a rejection slip.
Absolutely you MUST send a note of some form to every agent and editor you have received any consideration from. But they said it was not their sort of genre. So what? Say thank you that they took the time to read whatever you sent them and responded with more than a form letter. But they sent me a form letter rejection. So what? Send them a thank you for their time anyway.
Guess what. A form letter is no indication that they didn’t read it or that they are ignoring your offerings. It is not unheard of for an editor or agent to get a thank you for a work that they rejected and remember the name far more clearly than many of the manuscripts that cross their desks daily. More importantly it shows that you appreciate what they do and the time they take to focus on you regardless of what you get out of it.
Never underestimate how powerful a simple ‘Thank you’ can be. I have even heard of a few cases where an editor or agent was so impressed with the gratitude shown by an author that they made it clear they would be watching for anything this author sent their way thereafter and giving special attention to it. That doesn’t mean it won’t also be rejected, but it does mean that you aren’t just disappearing in the slush pile.
It takes a few dollars to buy a set of thank-you notes and a few minutes to write something in them. The cost of a stamp sends it off. Email might work as well, but many of them have special filters on their email so that anything other than submissions get sent into the spam or trash folders. There is no real substitute for a physical note.