Spammers prefer to send spam from legitimate email addresses and if possible they like the email look as if it came from a known person. This helps them bypass the spam blocker and increases the chances that the contact will open the email. The practice of making an email look like it comes from you when it does not is called spoofing. It is similar to writing someone else’s name and address in the “from” section on a postal envelope. If you are receiving a lot of bounced emails or people tell you that they've received junk from you, then you may be getting spoofed. You may want to consider setting up DKIM.