Email Spoofing
Spoofing is when an email header's FROM line is altered to something other than the email address it was originally sent from. A tell tale sign that you are experiencing this is receiving a large amount of Mailer Daemon/Failure Notification messages in your inbox. This indicates that the email spoofer set the FROM header to be your email address.
How Is This Possible?
Snail Mail and Email are not very different in that an individual can put any address for the return address. However, you can reduce the chances of this!
What Can I Do?
The BEST methods to help prevent spoofing is to create a Sender Policy Framework (SPF) Record, and ensure that DKIM is properly configured.
The SPF record will filter out unverified users trying to send messages from your domain.
The Domain Keys Identified Mail (DKIM) essentially adds a digital signature to your email's domain that recipient servers can cross-reference to ensure that the message is coming from you. As a result, this also helps reduce the chance that emails are getting marked as spam by the recipient mail server. 