Whenever I'm on the phone with a subscription or membership site publisher these days, one of the questions I invariably ask them is, "What email service provider are you using?" I can instantly tell from their answer how well run their email program is -- especially whether their email is actually getting delivered. (The latter is the primary, but by no means the only, reason most publishers should be using a good ESP rather than in-house systems.)Even with Twitter, RSS Feeds, online groups, mo...