Seems like responses are getting truncated after the first paragraph, at least via email. Trying again via the comment box on the site:
Do not build this yourself. Pay someone like Sendgrid, Mailchimp (Mandrill), Mailgun, etc. The cost/benefit is probably an order of magnitude in favor of a hosted service unless you're doing millions of emails daily.
To really build this yourself you would need to:
1) Hire a fulltime *experienced* postmaster that can run an MTA and has relationships with every major email provider
2) Buy MTA software, maintain hardware and IPs for it. You can't just host this "in the cloud" on open source stuff. Maybe, but it will be highly suboptimal.
3) Build and maintain a templating engine. Make sure the templates work on every mail client. (You'd pay ReturnPath for this probably)
4) Handle unsubs and comply with CAN-SPAM. That's a big DB of permissions to maintain.
5) Figure out a way to process all the feedback loops (hard bounce, soft bounce, etc).
You're basically looking at 2-3 highly experienced engineers / postmasters to get this up and running and maintain it. Why pay all that overhead instead of being up and running in under and hour with all of the above and pay-as-you-go?
I built the email system (transactional and blaster) at Zynga and even as a company of the size of Zynga three years ago, we went with external providers initially (pre SendGrid days - the providers were not very good...). Only once we knew we had to scale beyond the limits of what a hosted service could provide did we bring the capability in-house bit by bit. Even then, it took some time to justify building a marketing blaster. When we finally did, it was really not a cost reduction, but a "better product" decision.