There are a lot of problems regarding emails and graphic design. A lot of email systems will automatically block everything except text from being downloaded onto the computer, which prevents people from seeing the design in most cases. The only situation in which they will allow images is if you’ve been manually added to a safe senders list, and banking on that happening for your email marketing campaigns is pretty far-fetched.
Because of how the chances of getting your email read with all of the graphic design included in it are rather low, many people underestimate its importance. Regardless of how few people see an email you’ve sent without the graphic design, as emails are primarily a means of managing and developing relationships with customers, there is an expectation that they would eventually add your email to the safe senders list, and at that point the graphic design can make a lot of difference to how much repeat business you get from them.