1. A newsletter is not an ordinary e-mail
It has much more complex structure than a typical e-mail – it contains graphics, links ordered in a given pattern that determines what the newsletter looks like. In this case content editing is limited to arranging the content in predefined fixed areas.
2. A newsletter is not a website in an e-mail
Even though a website and a newsletter are both based on the HTML and CSS, in the case of newsletter the possibilities of that language are limited. For this reason many effects or solutions that we see on websites can not be transferred to a newsletter (e.g. interactivity).
3. A newsletter does not always look the same
This is because it can be open on various email readers (both typical applications and webmails), each with a slightly different range of HTML and CSS possibilities, and it often happens that they display the same code in a completely different way! You need to remember about this when designing a newsletter, and using your experience try to reduce the negative impact of those phenomena. You obviously also need to test it on the most popular email readers to find and correct possible incompatibilities.