As far as I'm concerned, the biggest problem is making sure that your email gets to people's Hotmail/Yahoo/etc. inbox, rather than their spam folder or rejected

As far as I'm concerned, the biggest problem is making sure that your email gets to people's Hotmail/Yahoo/etc. inbox, rather than their spam folder or rejected entirely. From my experience so far, this seems to involve messing with SPF/SenderID/Domain Keys/DKIM, filling out various forms, harassing Microsoft and Yahoo support, and sacrificing small animals at midnight. None of this adds any intrinsic value to a start-up, it's just a hassle that may as well be outsourced. I don't know if that's part of what these services promise, but it's the only thing that would make me consider using them.