Apart from cluttering the message up, this is another way that email addresses can end up on an infected computer for harvesting and used to propagate malicious content or spam.
Respect Privacy It’s also a way to give away a person’s email address to someone else on your mailing list without permission. In the illustration, there’s a whole list of names (the addresses can be seen with a quick mouse click) of people that I wouldn’t know from Adam. Supposing Felisa Stokes was my ex-partner and she wanted to remain non-contactable by me?
Clean up messages before you forward them! Doesn’t take much to delete the headers, addresses and anything unwanted at the end of the message before hitting the ‘Send’ button.