At the same time, if you are willing to install and manage Postal on your own servers, it's not that hard to maintain your own IP with a great reputation. You just need a good hosting provider (probably not AWS), you need to set up your infrastructure like DKIM, SPF, DMARC, rDNS, and Return Paths, and most importantly you need to maintain good engagement (low bounces, high opens). At a glance, Postal looks like a nice option if you want to do it on your own for cheap. You just might lack the stability, support, maintenance, and performance that goes behind an ESP.
This is probably a key element of good performance. To keep my mail admin duties part time I simply whois the IP of evil senders and drop the resulting entire CIDR block into the our local blacklist.
A couple years ago I did the work to use "Mandrill" in an app and they got merged with "MailChimp" who changed how my app could use their service and I had to redo those routines again.
Right now I'm using Mailgun and they're awesome.