Even though the world is opting to social media platforms for communication and all other types of advertising at the same time there is still a wide population that still uses email. It is still the most convenient mode of written communication. It could be marketing mails or a personal mail, the masses still prefer the email.
Designing an email web page is no different than any other regular web page on the World Wide Web. As a web designer it is important to think from the prospect of these individuals, whose web page time of viewing is limited i.e. few seconds.
Try making your web page a very responsive and user friendly page. Like reduce the layout between 500 x 600 pixels. The fonts should be appropriate for all users as long as the font size is not upscale. In general, concentrate on making the web page more eye catchy and user friendly.