E-Mail has been established as one of the most used services of the internet not only for private but also for business purposes. Thus, it is important to handle this stuff properly. I have listed the most common rules and tips.
Email Addresses
Most users only use the email-addresses that they get from their first provider (maybe AOL). If they change to another ISP, they lose their address and have to inform their contacts about the new one. A better approach is to use a freemail-provider like 
yahoo right from start. You can then use this email-address as long as you want! A good suggestion would be to use three addresses: One for important contacts, the second for mailinglists and newsletters and the third for newsgroups and spam distributors.
Spelling & Punctuation
At any rate you should stick to spelling and punctation: After each punctuation mark there should be a space but before there should not: It is false to write the way,I do now. But this was is OK, isn't it?
Hoaxes
You certainly get virus warnings or requests for emails that will make you very happy, save lifes or make the US-President to donate some money if the amount of emails reaches a special number. You are said to forward this email to as much contacts as possible.
If you you have a closer look at the content of these emails they have very often some locical mistakes: A lot of warnings about virus refer to well known software firms (Microsoft, AOL) in order to it make more credible.
In my experience most emails of this type were quite wrong, in other words: the emails were hoaxes. The authors aim to waste the resources of the users and the internet. Therefore, you should be informed before you forward such a email, e.g. here. And do not be fooled by the fact that you know the forwarder, since he maybe have not heard of hoaxes.
Viruses
Despite these wrong warnings there really are viruses that causes damage. Nowadays it is quite easy to create such viruses using special programs. Moreover, the monopoly of Microsoft-programs make it more easy to spread since their programs are in widespread use, and contain a lot security problems that are not quickly fixed. Furthermore, a lot of users do not care about security-fixes. Another problem is the default set of options that execute dangerous programs automatically or after a short mouse click. These are things that make it easy for a "worm" to read the address book of outlook and spread itself without the knowledge of the user.
You can escape this problem if you use better programs and if you do not trust each attachment.
Advertising, Spam
Spam is advertising that you get by email that you do not want. The sender of this email could have received your email-address from several possible sources: You may have give it to him yourself, it could be taken from a newsgroup or mailinglist or it could be generated automatically using a pattern.
If you get some spam, do not answer and do not follow any link that is included in the email, even if the email tells you that you will be removed. If you do that, you have verified the existence of your email-address and you will get a lot of more spam!
You can prevent spam in newsgroups by using an unused email-address as sender (from-header) and the real address for the reply (reply-to-header). Additionally, you should change this address regularly, let's say half a year.
Responses, Quotation
If you get an email, you usually reply to it. But a lot of users answer the whole email within one part before the original mail. This makes it more difficult for the recipient since he has to switch between his part and the sender's one. It is more readable to put put your answer after each part. And do not forget to remove the parts you do not refer to in this email. The character for quotation should be this one ">".
Mailinglists
In Mailinglists, emails are distributed to several people. If you answer such an email, it will be sent to all the others subscribers. You should pay attention if you want to send an email to a single person, since you have to hit the "reply to sender"-button. Moreover, you should not send big attachments to the list.
Attachments
You can send a file as an attachment by email but you should be careful of filesize, since the files are bloated with a factor of about 1.3 due to a special encoding. Thus, do not send big attachments by email. A size of half an MB is still OK but emails that are bigger will not fetched by me.
Signature
The signature is at the end of an email and can contain your address or sayings. It it separated from the the email-text with a "-- " ("dash, dash, space"). Using this sign the recipient's email-client recognizes it as a signature and should display it in a special way or delete it when you reply. Wrong separators are 40 dashes or so. Additionally, you should not use more than four lines as signature.
Encryption
Due to the widespread use of email, a lot of information is exchanged including some that is very private. But few people know that it is no problem to read and modify the email while it is being transferred through the internet. Thus, you cannot be sure that the email you have received is really from the sender and that it has not been read and modified!
But you can encrypt and sign your emails: In the first process the email is encoded so that only the real recipient can read it, in the second one there is calculated a special checksum by both the sender and the recipient; if they are the same, the email has not been modified.
For these purposes PGP, or his OpenSource-cousin GnuPG, has been established. Using these and the right email-clients you can communicate in this secure manner. I have created a list that contains (I hope all the) email-clients that support the better PGP/MIME-standard.
