SendGrid ftw

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SendGrid. It handles all the crap for you, even lets you setup a whitelisted/rdns domain if you want to ensure maximum email delivery (which the others don't). Plus there are already rails apps coming out on github to track stats independently from the sendgrid website... it's all on heroku.

Incidentally if you happen to be using heroku to write your rails 3 app they've got an add on for sendgrid so you can just integrate from there. We try to make your life easy :).