1) Sending too Often
Most business owners think “more” is better, they often over send the email to their subscribers. Ideally, one email subscriber should only receive one email from you per day, this is the maximum that I would recommend, alternatively one email every two days will work fine as well. However, I encourage you to always analyze the result, you can use an online tool called Sender Score (https://www.senderscore.org/) to check your email deliverability if most of your email started to land into spam box even though you are sending once per day try to reduce to one email every two days.
2) Not Analyzing your Metrics
Not analyzing your metrics is like gambling your result. The main metrics to look for are open rate and click-through rate. You may find that one subject line gets the highest open rate, but another subject line gets greater click-through. Therefore, take note of those when comparing your emails results.
3) Being too General
A research study found that personalizing your email can increase open rates by as much as 137% and click-through rates by as much as 129%. While these numbers may vary from study to study, they are still indicative of the importance of writing like you are talking to one person.
4) Not Checking Spam Score
It is a waste when your subscriber never receives your emails in their inbox because most of your emails land into spam inbox. Most email auto-responder have the feature of letting you check the spam score. One tip to reduce spam score is to reduce your email copy curse words and use a verified domain email.
5) Complicated Template Design
When it comes to template it is important to keep it simple, for example, most people tend to use a lot of image in their email newsletter, it will actually cause Gmail to filter your email as a promotional email, one image in one email is the maximum.