DON'T OPEN UNEXPECTED ATTACHMENTS.

If you don't open an attachment, it can't infect you (unless it's found a security hole in your email client). Many people fall into the habit of opening attachments without thinking about it. Don't do this; you should always think about whether you want to open the attachment first. Assume an attachment is hostile until proven otherwise.

Practically speaking, of course, sometimes you are in a position in which you have to open attachments. In that case, you should only open it if it passes through all the other guidelines, as well.

• DON'T OPEN ATTACHMENTS FROM STRANGERS.
  Ever. If you aren't absolutely certain you know the person who sent you the attachment, don't open it.
   

• DON'T OPEN UNEXPECTED ATTACHMENTS.
  If you do know the person in question, but weren't expecting them to send you an attachment, contact them and confirm that they sent it before you open it.
   

• DON'T OPEN UNUSUAL ATTACHMENTS.
  Most of the attachments you receive probably fall into a few recognizable categories -- Word documents, Excel spreadsheets, PowerPoint presentations, and/or Acrobat PDFs, for example. Learn to recognize the icons and filename extensions associated with these files; if you receive an attachment that has an unusual icon, or an unusual extension (especially ones like .pif, .scr, .com, .zip, or .exe), don't open it.
   

• DON'T OPEN ATTACHMENTS FROM STRANGE-LOOKING MESSAGES.
  If you receive several unexpected messages from different people with identical subject lines, it could be a virus or worm. If the subject line or message body before the attachment seems unusually vague, incoherent, or incomplete, don't open the attachment. If it just looks "wrong", don't open the attachment.