From where I sit, both of those concerns sit on the same side of fence. GP argues against extensive developer time spent on validating edge-case emails, and says they do so in no small part to avoid having emails bounce etc., as doing MX or other validation to follow-up on these edge-case emails validity within your service does nothing to imply others have put in this same costly and nearly superfluous support, likely leading to more emails bouncing and accordingly degrading the trust in their business as a sender