Mail messages that appear to come from you or someone in your organization...A likely possibility is that you have been joe-jobbed... more or yet more

Check the originating mailserver in the message headers... if it's not yours, you did not send it. Unfortunately, there's nothing you can do about it. For most - unlike the gentleman at the first link above - being joe-jobbed is annoying, but essentially harmless.

You can take greater measures if you find that you're unlucky enough to have a bunch of these messages go to members of your company's address book(s). Look for virus or worms on individual desktops that have been compromised. This is an issue well beyond the joe-job concept. Call tech support for further investigation.