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. If you do not want your message forwarded to anyone else, say so. The convention on mailing 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 to use the telephone and to write letters as small children.
Appropriate phone or letter etiquette is second nature to most adults. As a result, they make mistakes. Most people, if told politely, will be happy to follow the conventions. They just need to know what the conventions are. Use With Care E-mail can be a wonderful communication tool when used with care.