IDEA shows it as non-html attribute and could be deleted.
After a bit of googling I figured out that that attribute is only used by MS Outlook.
As images can be a part of email message or regular hyperlink to an external image MA decides how to handle it.
When you are forwarding/replying to a message regular MA leaves it as is. When MS Outlook tries to rewrite them somehow, add as an attachment or a part of mail message. So it copies your images and rewrites URL, when probably you don't Outlook doing this. That's fucking strange, so generally speaking if you do not want to include images into the message (as a part of it) add attribute NOSEND="1", otherwise it will be bundled them as attachments to your forwarded/replied email message.