Emails Look Bad
Email clients (Gmail, Apple Mail, Thunderbird, Outlook, …) sometimes have trouble showing the email content correctly formatted. The Newsletter plugin does the best to generate layouts that are correctly shown on those clients but Outlook makes life harder for every platform and Gmail ignores the mobile style for non-Google email accounts.
Be sure to not copy and paste from Word in a newsletter text block since most of the time it contains hidden style rules that do not look good on email readers.
If images are not shown, check on top of the email if the reader is alerting about blocked images: if so, there is nothing Newsletter can do to solve that problem.