Even in the relatively compressed form of an email, you must explain your ideas adequately; in other words, you must develop them. The reader should not have to work to understand what you mean; rather, you — the writer — should do the work for the reader.
Thus, complex points must be allotted time and space sufficient for the reader to understand them. General statements should be supported with details and examples.
Consider the following paragraph from a hypothetical email message:
We must do a better job of meeting customer needs. For example, we have to respond more quickly and professionally to customer requests for assistance.
At first glance, the paragraph may seem to contain the supporting details necessary to explain its main idea, that we should do a better job of meeting customer needs. What, however, does more quickly and professionally mean?
Now compare the paragraph below with the preceding example.
We must do a better job of meeting customer needs. When a customer calls or emails with a problem or complaint, we need to resolve the issue within 24 hours. In addition, we should follow up with a phone call within five days of an initial inquiry to ensure that the customer is satisfied with the resolution.
The revised version contains concrete, useful details. Although the first version did not develop its main idea, the second does.