Getting email to display correctly on mobile is no simple task. Making email "responsive" - meaning ensuring messages display clearly on any viewport size - is now the current equivalent of what making the web responsive was a while back. Unfortunately, many emails still do not display well on mobile. Which means a large amount of potential audience impact is lost.
Note this is a "silent error". You send an email and all seems to go well. The reality is, many recipients may open the email but then delete it when they cannot read on mobile. The potential value of reaching your audience has not been realized.
Here is how an email from Finborough Theatre displays on iPhone6s using the Apple Mail client.
Examples of good email design
Below are a few examples of responsive emails that are properly formatted for mobile.