Like everybody else on the web, I often receive messages from somebody who is responding to an earlier email – whether it be mine or someone else’s. Even if I’m the original author, I don’t always remember what the 1st message was about, especially if it’s not answered immediately.

For example, when I receive, “I don’t know who that is! It looks like you caught him while he was blinking.”, I assume it’s referencing an image from an earlier message – but which message?!

So, if you’re not going to reply with the original attached file, which totally makes sense in order to cut down on bandwidth concerns (although it usually doesn’t apply anymore, but still feels good to keep internet traffic short & succinct), at least include the name of the image, video, etc. you’re talking about in the subject line or body of your message. That not only saves even more bandwidth, but also time and confusion for the receiver.

btw: this article on email etiquette lists 25 more email etiquette items you may want to take a quick look at.