It's important not to confuse email marketing best practices, and email best practices in general, with email "common practices".
In fact, this can be a very dangerous, slippery slope on which to pin your email deliverability and indeed your email reputation. In fact at one point in time there was a meme going around suggesting exactly that, that it's "important to follow email marketing common practices." No, no, NO! To suggest that best practices aren't really best practices, but only 'common practices', reduces them to "not necessarily required" practices.
