What you need to remember when designing a newsletter:
– width: the maximum safe width that guarantees the newsletter will be visible without scrolling is 700px.
– small number of fonts: for text elements use web-safe fonts. You can also paste some texts as graphics – a graphic can contain any font. However, this solution has other limitations. See below.
– impossible to add text elements on an image (image as background): you can solve that by placing text + background as one image.
– impossible to add videos, dynamic or interactive elements and JavaScript: you can partly compensate that with animated GIFs.
– legal requirements: users must be able to sign out from a newsletter
– responsivity: more and more users use mobile devices with smaller screens. It is worth designing a newsletter that adjusts automatically to a smaller width of the screen (responsive).
– spam filters: some solutions (e.g. too many images with not much text) increase the risk of being qualified as SPAM, thus decreasing the chance it will be seen by the reader.
– size: large newsletters take long to download (this can be a problem particularly for mobile devices with download limit) and can slow down your mailing system.
– attaching images: in order to limit the size of a newsletter, you can not attach images to it but put links to images on the server. But this solution may cause the images not being displayed without the user’s permission. In that case the reader will only see the text elements with empty squares of the lacking images.
– editing of sending newsletters directly from email clients: very often email clients interfere with the HTML and CSS in a way not visible for the user, making changes to a properly prepared template.

Here are a few examples of newsletters from Freshmail professional newsletter software:
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If you want to minimize the risk of problems with newsletter display on various email clients, use a carefully developed and tested template. In consecutive issues you add or remove predefined sections, replacing the content of defined areas (fixed size images or texts). It should be sent from a dedicated app, which minimizes problems and provides statistics. But newsletter statistics are a separate subject…