Where's your evidence that anyone has better deliverability than SES?
1. Infrastructure. A lot goes into this, and you can Google what needs to be done, and do it yourself if you have the time.
2. Content quality. If your recipients keep flagging your mails as spam, nobody will help you - that's your responsibility not to spam people.
Myriads email sending services do #1. Mailgun, among many other things, does that too. Plus, we can give you a completely separated, dedicated setup where your reputation won't be affected by other senders (something SES can't promise you).
But in the end, you need to send something people want to read. Saying "email company X gives you automatic deliverability" is a bit like saying that "hosting company Y gives you automatic PageRank" :-) Yes, your emails will be 100% solid on protocol/format/rate level and the IP you're sending from is watched, but you'll be taken down if you're spamming.
Hope this helps. And if you need assistance regarding deliverability (on your own server, or with Mailgun) - shoot me an email at ev@mailgun.net BTW, this mailbox is hosted by Mailgun itself. see? we're eating our own dogfood here!
Ahh, so you're using a different IP / subnet for each customer, then? With the internet running out of IPv4 space, I think that companies that do this kind of thing should just have all their addresses revoked. Same goes for "SEO web hosting"