They're in the commodity e-mail delivery service too -- Mandrill -- in addition to MailChimp. Pretty smart to reuse the hardware, staff and ISP/RBL relationships they established with MailChimp to also serve the SMTP-as-a-service market.

I use both. I have sendmail on my servers set to route everything through Mandrill, so my apps don't need to be aware of what software/service I choose for e-mail delivery. I send newsletters/product announcement type mails with MailChimp for the design templates and subscription management.

One could argue that services like Amazon SES make email delivery a commodity (though I'm personally not convinced of that).

But that's not the market Mailchimp is in. There's a lot more to sending newsletters than an SMTP server. Amazon SES is not their competition.