Final answer:
Letters should be used when formality and sensitivity are essential, when immediate feedback is needed, and when there is too little time to proofread an email.
Step-by-step explanation:
Although email is successful for both internal and external communication, there are still certain situations where letters should be used instead. One of these situations is when formality and sensitivity are essential. For example, if you are writing to a company, politician, or institution, a business letter is more appropriate than a casual email. Another situation is when you need immediate feedback or the message must reach your reader very quickly. In these cases, picking up the telephone or speaking to the person in person would be more effective than waiting for an email response.
In addition, if you have too little time to proofread your message, it is better to compose a letter. Unlike emails, which can be sent instantly with a click of a button, letters usually require more time and effort to write and send. Therefore, by dedicating the time to write a letter, you can ensure that it is error-free and conveys your message appropriately.