If your mail system allows you to forward mail, beware the dreaded forwarding loop. Be sure you haven’t set up forwarding on several hosts so that a message sent to you gets into an endless loop from one computer to the next to the next.
Don’t send large amounts of unsolicited information to people.
Know how large a message you are sending. Including large files such as Postscript files or programs may make your message so large that it cannot be delivered or at least consumes excessive resources. A good rule of thumb would be not to send a file larger than 50 Kilobytes. Consider file transfer as an alternative, or cutting the file into smaller chunks and sending each as a separate message.
The cost of delivering an e-mail message is, on the average, paid about equally by the sender and the recipient (or their organizations). This is unlike other media such as physical mail, telephone, TV, or radio. Sending someone mail may also cost them in other specific ways like network bandwidth, disk space or CPU usage. This is a fundamental economic reason why unsolicited e-mail advertising is unwelcome (and is forbidden in many contexts).
“Reasonable” expectations for conduct via e-mail depend on your relationship to a person and the context of the communication.
Norms learned in a particular e-mail environment may not apply in general to your e-mail communication with people across the Internet. Be careful with slang or local acronyms.
If you think the importance of a message justifies it, immediately reply briefly to an e-mail message to let the sender know you got it, even if you will send a longer reply later.
Just as mail (today) may not be private, mail (and news) are (today) subject to forgery and spoofing of various degrees of detectability. Apply common sense “reality checks” before assuming a message is valid.
If you include a signature keep it short. Rule of thumb is no longer than 4 lines. Remember that many people pay for connectivity by the minute, and the longer your message is, the more they pay.
Mail should have a subject heading which reflects the content of the message.
Limit line length to fewer than 65 characters and end a line with a carriage return.