Email marketing can be a tough gig.
To successfully reach their goal (you do have goals for your marketing campaigns, right?) the savvy marketer needs to reduce the effect of multiple barriers to entry. Think of it as a big funnel that gradually reduces the volume of visitors at every stage:
Does the content bypass spam filters and work reasonably consistently across most email apps? Did it get opened? Did it get read? Did viewers click through to the website? Did they perform the action you desired?
At every point you need to maximise the number of viewers making it through to the next stage in the funnel.