Here's a list of some common errors I've seen people (including marketing people) make in their email marketing:
1. Using underling for emphasis. This confuses users, as users have become accustomed to underlines being used for links. They'll try to click on the underlined text and think it's a broken link. It may cause them not to bother to read on.
2. Use of "Click here" for links. For accessibility purposes most companies stopped using "Click here" in digital communications several years ago. In fact, Nielsen Norman Group advised against using "Click here" as early as 1996. If you are still using it, it makes you look a bit dated.
Best practice recommends using descriptive links. For example hyperlink "Contact us", not "Click here" to contact me. Read more about "Click here" on Nielsen Norman Group's website.
3. Using bold for links in one place and for emphasis in another. The inconsistency confuses users and they won't understand what is clickable and what is not. Again they may not read on.
4. Overusing italics. This may include long passages in italics, such as customer testimonials. Italics are more difficult to read online and should be used sparingly.
5. Assuming everyone will download the images. Various research reports suggest between 40% to 50% of email users do not download the images. Are you placing important copy and calls to action in images? Do you check how your email will read if the images are not displayed?
Check your email marketing for these errors and see the improvements in conversions. Alternatively, get in touch if you'd like me to review your email marketing.