For myself, I greatly appreciate having a service like Mailgun available to Rackspace customers - does Linode (or another company) have a similar e-mail service?
If you're using it for outbound mail, you might look at using Mandrill. They're much bigger -- it's MailChimp's backend, used by 5M companies. That means mail is their primary business, their ISP relationships are solid and you can count on them to have 24/7 staff if anything does go wrong. They're also quite competitive: 12K free e-mails per month, then cheaper than both Mailgun and Sendgrid.
You don't need Rackspace to use mailgun, but Sendgrid also works nicely and is independent. Just like you don't need to use EC2 to use S3 (or Route 53, etc).
http://www.mandrill.com/