How it works?
First you should have a POP3 enabled email account. If do not have POP3 enabled email account, you can create one from Gmail for free. Set up your POP3 account to work with Outlook and I am not going to tell how to set up an email account in outlook here.
Then you need to do a simple change which will let you email from any email you want to.
Go to Tools >
Email Accounts >
View Email Accounts Details >
Then change your name and Email Address as shown in the picture.
How simple is that. I think this is not a big security issue but someone can misuse it. Please do not misuse it but you can have bit of fun. This works in all Outllook versions.