Ever see an email from someone at your own domain, yet you know the person is not a part of your company? Chances are it is is commercial spam–a “joe job”–and if you don’t open the email, it is relatively harmless.
According to Wikipedia, a joe job is a spamming technique that sends out unsolicited e-mails using spoofed sender data. Early joe jobs aimed at tarnishing the reputation of the apparent sender or inducing the recipients to take action against the person, but they are now typically used by commercial spammers to conceal the true origin of their messages.
If you want to determine what to do, you can look at the code header of the email to confirm the sender is really from another domain. Don’t open the email, though. To look at the header safely in Outlook, right click on the email sitting in your inbox and select Message Options (see image 1 below).
In the line “Received: From” you’ll be able to see the true origins of the sender (see image 2 below) and it will most likely be somewhere other than your own domain server. Simply move the email to the junk folder and forget about it.
If by some chance, the sender origin really IS your own domain then contact your hosting company immediately! A hijacked email account is a serious problem and should be dealt with quickly. Needless to say, to reduce the chance of someone accessing your account and taking it over make sure your hosting account passwords are complex.