This rule resonates with me because sometimes when I am not in a normal mood,
I post something in public without enough thinking which ends up taking down.
I personally find this quote quite funny, and at the same time, it makes me think how vital our internet world has grown up. When we first got introduced to the whole called internet (I think I was around an elementary year), we didn't take that so seriously and end up creating a very personal and unique ID for emails. However, nowadays our email address became part of our identity which needs to take seriously.
This is something I always have in my mind. I don't think any other explanation would be needed for this specific quote. :)
<images are from eEtiquette.com>