Google's Gmail might be the preeminent mailbox provider. Launched in 2004, Gmail has grown from the "new kid on the block" into one of the biggest hosts of individual email mailboxes in the world. Depending on what data you look at, you might even see Gmail as the #1 mailbox provider, at least here in the US.Gmail's spam filtering systems incorporate user feedback and engagement. And they know what they're doing. If you are not sending wanted mail to people who requested that mail and who read that mail at high enough percentages, you're going to struggle. You won't reliably...