Sending attachments
First, a few words about file extensions. These are the letters that appear at the end of the name of all the files on your computer.  You will see that some are .doc or .txt (document or text file) others may be .html or ,bmp or any number of other things.  Not all these are a good idea to send as attachments., for various reasons.  The number one NO-NO is any file with the extension .exe  These files may well be benign but it is .exe files which commonly carry viruses and they are ones to be very wary of.  Do not send them on without reading these rules and thinking about it.  And if you receive a file with .exe after its name do not open it unless you know what it is and you are expecting it.  I cannot emphasise strongly enough that it is not good enough for you to say - it must be ok, it is from my mother.  The fact is that it may be a virus that has got onto your mother's computer and without her knowledge is sending itself to everyone in her address book.  DO NOT OPEN IT unless she has told you it is coming and ascertained that you want it.  It is safe to send on files with the extensions .jpg and .gif - these are pictures.  It is also safe to send on pictures with the extension .bmp but they tend to be pretty hefty files and your friends will not thank you for it - better to convert them to .jpg before you send them.  Word documents can contain macro viruses and are not a good thing to send on or to accept when people send them to you.  A far better idea is to copy and paste the contents into an email - use rich text if you do not want to lose the formatting.  Educate your friends about what is and is not acceptable!
You can look up file extensions to see what they are.
Just remember that NO JOKE, NO CUTE SENTIMENTAL PROGRAM, NO RUDE VIDEO is worth risking your computer files and peace of mind for.
Here are some basic rules:
Before you send anything it is sensible to find out if the recipient has the equipment to view whatever it is. It is pointless sending a file that your friend cannot look at.
Never send executable files - those with a .exe extension - without finding out ahead whether your friend wishes to receive them.
Never send a large file - anything over 200kB without finding out first whether it is convenient for you to do so:
1. It holds up all  important email while it downloads slowly - your friend may bein a hurry and waiting for something of importance.2. He may have seen it already and be pretty annoyed at the waste of timereceiving it again....3. He may be paying a lot for a timed internet connection and do not want tospend that precious time waiting for some *(&*^&^  to come in!!!
4. He may have an account that limits the size of his inbox, and if you fill it with junk he will not be able to receive his more important mail.
Delete others' email addresses
This relates to a common problem when people send on jokes or info that they have received by email:
One of the first things to learn is never to send other people's email addresses aroundthe world. Whenever you send something on, delete all the guff - otherwiseeveryone's address gets sent on and on until it is way out of your circle ofacquaintances - you don't know who will get it and use it for all sorts ofthings.

This is a real NO-NO and is extremely discourteous to whoever sent it to you.

Now that I am in my stride, one more thing!  I usually refuse to read an email with >>>>>>>>>>>>> all down the side - I just delete it. If the sender cannot be bothered to clean it up I can't be bothered to read it.
If the email comes in "clean" and you want to send it on, don't use the Forward button - that will add >>>> to the message.  Simply copy and paste the item into a new email.
If the email already has the >>>>>> down the side, either use the Find/Replace in Word or some other word processing program, or download a simple program which will do it for you.
There are lots of these. 
You can download Email Stripper from PaperCut or Ecleaner from PCWorld
or there is a neat little online Stripper (which you can also download) at Mr Ed's
If you ever send an email to lots of recipients, either use a "group" nameto send it to, or put all the names into the BCC   line so nobody knows whoelse got the email. I send news updates to many people every day and allnames are in bcc. If you are using Outlook Express and do not have bcc showing on your email form at the moment go to View and see that All Headers is ticked.