My business relies heavily on e-mail (we send e-mail proposals on behalf of hotels to their customers), and we use AuthSMTP (http://authsmtp.com). $168/year buys you 10000 outbound e-mails per month.

That said, guaranteeing 100% success with the delivery of e-mail is pretty much impossible. You just can't control what the recipient has installed in terms of mail client, spam filtering software.