Integration is becoming such an important factor in the sales and marketing process that the average email program simply isn't enough. Folks like Right On Interactive have taken their marketing automation system and built their own email platform directly into it so you need not purchase or integrate and ESP.
It's not difficult to actually do. When he was frustrated with the limitations of email services, Adam Small utilized open source mail transfer agents (MTAs) to construct his own email platform integrated with his real estate marketing platform. He now sends millions of emails on behalf of real estate agents with high deliverability scores at a fraction of what he used to pay.
There are hybrid applications out there now, too. Mailchimp launched a service last year called Mandrill. It's basically the back-end of Mailchimp with all the reporting, deliverability and support… but with only a comprehensive application programming interface (API) rather than a front end.
Simply put, you build your application and plugin to Mandrill to do the transactional email sends. Many, many IT departments make the mistake of sending email utilizing their servers' SMTP engines. The problems arise almost immediately… between spam reports, bounces, feedback loops, authentication and sender scoring, it's almost a miracle that your email makes it to any inbox. Any company sending thousands of emails needs to manage the deliverability process by utilizing the acceptable technology.
Pricing is very good… send up to 12,000 emails a month for free! Send up to 40k emails for $9.95/mo. Prices move per thousand after that and stay very competitive.