So not only could you choose (swap out very very quickly) your outbound provider, but also your inbound provider.
After the thread the other day on outbound email, I ended up choosing MailJet who have been wonderful with the questions I had when I hit their new member ceiling within the first few hours.
The things that convinced me on MailJet were real-time outbound stats and meaningful data (recipient + subject), API of status notifications (who bounced), and the fact that they do campaign emails at a cost so low that MailChimp ruled themselves out (one of my email lists has over 23,000 members).
I need inbound handling (currently using /etc/aliases and piping), but would love all the reporting and info which centralisation from an external provider could deliver.
My only concern would be resilience. Email is really good at re-delivery when your end is down or something prevents delivery, would a HTTP REST interface offer the same level of re-try and timeouts to ensure it got through eventually. Would the provider queue for me?