[email] How do you make sure email you send programmatically is not automatically marked as spam?

You need a reverse DNS entry. You need to not send the same content to the same user twice. You need to test it with some common webmail and email clients. Personally I ran mine through a freshly installed spam assassin, a trained spam assassin, and multiple hotmail, gmail, and aol accounts.

But have you seen that spam that doesn't seem to link to or advertise anything? That's a spammer trying to affect your Bayesian filter. If he can get a high rating and then include some words that would be in his future emails it might be automatically learned as good. So you can't really guess what a user's filter is going to be set as at the time of your mailing.

Lastly, I did not sort my list by the domains, but randomized it.