For me, it's more the support you get from using third party APIs. It's nice to break away from companies to a certain extent, but if you need help/support you only have yourself to fix the issue. If you go with a third party and something happens, you can go to them for a solution.
And if the third party service goes down, you're SOL -- there's nothing you can do to fix the issue, and it's your users who are suffering. What's your answer when a customer says "my emails aren't getting through, please fix it" and you...can't?
If emails are a part of your core business, you should have a business continuity plan for this situation. That may be to fall back to another service, or even a temporary in-house server.
Of course, if it's that important, then you'd be foolish to not have invested in a better solution, or to not have a proper SLA with a third party.
This is a major risk of outsourcing part of your operations. You probably
shouldn't do this to the core of your business, as you lose control over
problems. But sending marketing e-mails usually is not the core of business,
unless one is selling the service of sending e-mails.
