You can achieve a kind of immortality through your e-mail well out of proportion to the amount of effort it takes to send it. It can be a good idea to explain your intentions to the recipient of a message. For example, if you do not want your message forwarded, say so. The convention on discussion lists and Usenet newsgroups is that private e-mail should not be publicly posted, but people are occasionally thoughtless or unaware of the convention. And you can wind up defending them to people you never thought the message would reach.
Most people learn as small children how to use the telephone and to write letters.
Appropriate phone or letter etiquette is second nature to most adults. Most people on your campus, however, have had electronic mail for a much shorter time maybe one to five years. Many incoming students have their first experience with e-mail during orientation. Electronic mail is a very new method of communication for most of the people who use it and they're still learning the ropes. As a result, they make mistakes.