Get to know the audience of once:

The presentation and content of an email marketing message or campaign often depends on whether the objective is to acquire new customers or encourage loyalty and repeat purchases from existing customers.

Acquisition efforts are more likely to focus on encouraging action, retention efforts on building relationship making the opening announcement that greets recipients once they have opened the email.
Before you start writing, be sure that you have a clear understanding of your audience. Ideally, this helps communicates the company's unique selling proposition and encourages the recipient to penetrate further into the email.

Effective content is engaging, useful, informative, educational, professional and entertaining.
Each email, as well as the overall campaign, should have a clear goal.

While sending a customized mailer for any product or service campaign it means you are addressing a niche group, the more you tailor your text to particular group, the more they feel relevant.

Another interesting factor while writing an email is, you can simply link to various sites by providing external links about products and services.

Links are information, and providing them could be appreciable. You can give your reader the result links to survey more for in-depth figures. To be effective in motivating action, links must be obvious to the visitor or recipient. When images or graphics are used as links, or when hyperlinks are used, always provide a corresponding text link as well. This helps keep it short and readable.

Color Theory
Creativity is about harnessing change. The content can be made colorful to help create an appropriate psychological state and present information most effectively, in addition to engaging prospects and directing their progress through the process. The degree to which design, layout and functionality are appealing to prospects and fits the "image" the business is trying to portray.

The email body should not be too long as it can have an adverse effect and the persuasion factor can get the customers annoyed. Long sentences can be confusing. In general, shorter sentences best capture and retain a reader's interest.

Its about having an optimistic approach thus the last sentence of the communication, must reinforce desire to take action.