What additional value does an email marketing service like Cheetah Mail provide over less expensive providers like MailChimp and SendGrid? (esp. for larger companies)

I think each "large" ESP has their own set of additional values over smaller, less expensive, email solutions. In some cases these ESPs focus on more full-service (or heavy services-based) engagements where their experience and expertise is helps drive results. You see this with big retail and e-comm, and with ESPs such as Cheetah, eDialog and Responsys. But this is very expensive and typically starts at $100-200k. Other general areas I believe the larger ESPs offer greater value include: - Infrastructure: provides greater reliability, uptime and throughput - Features & Functionality: it's not just about having a cool feature but being able to have many of them that will work effectively, and reliably, for all users. - Integration: Many ESPs offer APIs. The larger ones have more productized (and experienced) integrations with things liek web analytics, e-commerce, CRM and more. - Security: with the Epsilon breach excepted, when you are dealing with large financial organizations for their mission-critical customer communications you have likely been "audited"; moreso post-Epsilon breach - Support: 24/7 support is required if you operate globally; quick response (not 24-48 hours by email or callback) is also important The key for any organization when choosing an ESP is understanding where they are with their email programs, where they want to go, what integrations are required and what resources they have. Only then can you start looking for the best fit.

If email is not a large part of your core strategy and investment (including time) then Constant Contacts and Mail Chimps of the world are perfectly fine.  However if you're going to make a big investment in email, then you need to be equipped with the proper tools.  It's like seeing a contractor walk into your house with a Black & Decker instead of a DeWalt.

Craig is right. Services is the biggest differentiator. However in my experience smaller, full-service ESPs often provide greater value than their larger cousins.