Response to Your Email Signature Is Awful:
Incorrect. “Sent from my phone” is just fine. In fact it is a courtesy in two ways: (1) it lets people know that you’re trying to get back to them quickly because they’re important, and (2) it is indeed a sort of apology for any lenience in your punctuation, grammar, syntax, etc., which everybody understands will happen if you’re poking at a touch screen while between tasks. Of course one should not respond to all emails this way. Judging which ones can merit this type of treatment is really the only consideration.