It's not even necessarily being originated from your account, it could be the account of someone you sent a mail to once upon a time, or someone who sent you a mail, or a mailing list you're on. The malware will harvest all the contacts in a compromised account and then spoof the headers of the mail it sends to look like it comes from one of those captured addresses so the recipient is more likely to open it. If it's rejected for any reason by some mail handler on the way to the recipient, the error message comes back to the spoofed orginating address - ie YOU.
It's kind of scary how many companies, websites/forums and mailing lists are compromised - I give a unique mail address to every site/person I correspond with so I can track where things come from.
And it's still a good idea to lock things down - turn off automatic image opening on your incoming mails, be REAL careful what attachments you open, don't ever use any Microsoft program to handle your mail (Outlook and it's ilk used to be bug filled infection vectors, I hear it's somewhat better now but why risk it?), look at email first as plain text and only after you're pretty sure it's legit possibly switch on the fancy HTML formatting. (Lots of junk mail embeds unique id's in the addresses of the images in the mail, so just OPENING the mail and displaying the image confirms that it was sent to a valid email address AND that you opened it - just exactly what they want to know to send you more junk and sell your now-confirmed address to every other spammer under the sun).