Just don't understand why you don't operate your own mailserver?
Because having our own mailserver you will be blacklisted by other mail hosts quite quickly. We're letting a third party deal with our emails because they have a good reputation, and support DKIM/SPF validation.
This makes sure that our emails do arrive. For notifications it's not the worst if they don't arrive, but for signing up and password retrieval, this is crucial that it works.
There's also the part that... running your own mailserver is usually not a problem with your provider. They usually have a limit of 2500 emails per month or so. We yesterday, we sent out 2500 in one day! Mostly because of our notification system. And that's where they block you and tell you to find a paid service instead. You would think that this comes cheap, but it actually doesn't. That surprised us too.