The landing page your are sending your engaged eMail subscribers to is just as important as the email itself.
Is it optimized for mobile?
Does it have detailed information for conversions?
Do you know how your eMail subscribers are interacting with this page?
Are you A/B testing the landing pages for optimal conversions?
As you can see, there is just as much to do with the landing page as their is with the email. They both go hand and hand and should be viewed as one for the customer experience.