Email marketing is a proven effective channel for branding your business but there are many wrong techniques and pitfalls which can impact and bring harm to your campaigns.
Here are the list of mistakes that you need to avoid:

Over-hype Sales Pitch
Pushy sales copy, like “BUY NOW!!!!” or “LIMITED TIME OFFER or even over hyped claims like earning 10K in one day will devalue the quality of your email! You don’t want you email promotions to be seem as SPAM and junks by potential clients and subscribers. All it takes is one over hyped marketing email to tarnish your image as a value added honest online business owner or Internet Marketeer.

People May Forget Who You Are
People tend to forget newsletters they signed up for after awhile in particular if you have not been actively sending out regular newsletter to your subscribers or they have not buy anything from you. Always be reminded to include your business name, your contact email address, link to your business homepage and the mission statement of your business as well as a reminder statement as to since when the reader has subscribed to your email newsletter.

No Call To Action
A common mistake that will surely impact your campaign is no clear follow up action as to what you want the subscriber or email client to do at after they have finish reading the content. You need to provide a clear call to action by including a link to appropriate web page.

Ineffective Follow Up Link
It is important to take note that another pitfall that many people get into is providing a link to their main business web site home page instead of a landing page that is relevant to the email marketing campaign which they are promoting.
You don’t want your email clients to become confuse and expect them to have the patience to navigate to the landing page pertaining to the product that you are promoting via your email campaign.

Writing subject lines in All CAPS
Email subject lines in big capitals is not only unpleasant to the eyes moreover it will trigger the email SPAM filter. Furthermore, it is simply not appropriate to capitalize every single word as it will dilute the message you are trying to communicate with the recipients.

Excessive use of exclamation and other inappropriate characters
Avoid the urge and inclination to include unnecessary exclamation and other inappropriate characters such as smiley characters because it makes your email less professional.
Some people may argue that exclamation and emotion characters helps to make it more personal and resonate with recipients. Conversely, excessive use of such characters may cause recipients to over look the significance of the business message that you wants to communicate as people read between the lines and sees it as less business and more entertaining instead.

Conclusion
There are many more email marketing mistakes that you need to avoid apart from those that are highlighted as per above. Do check out the previous post on avoiding using irrelevant subject lines in email.
Email marketing is still very much a proven effective marketing strategy to reach out to new potential clients and prospects. As the saying goes, the money is in the list thus do no not waste all those efforts of yours by avoiding those mistakes described.