Microsoft Outlook uses a rectangular bar above the message header to display information and warnings about the message you're reading. It may report that you replied on such a date, that an attachment was suppressed, or that line breaks have been removed. Sometimes, though, you might see a very different message—perhaps something like "This message violates the USA PATRIOT Act" or "ERROR: Microsoft Outlook detected. Get a REAL email client!" As it turns out, anybody who can control the custom headers for outgoing e-mail messages can cause such a warning to appear. E-mail server administrators have this level of control, as do spammers, hackers...and you! Here's how.
Outlook itself doesn't offer sufficient control over e-mail headers, so your first step will be to install and configure the free Mozilla Thunderbird e-mail client. Now launch Windows Explorer and navigate to the folder C:\Documents and Settings\username\Application Data\Thunderbird\Profiles, replacing username with your actual username.
Within this folder, you should find one other folder whose name is a random collection of letters and numbers followed by .default. Open this folder and look for a file within it named user.js. If that file is not present, right-click in the right-hand pane, select New Text Document, and rename the resulting document to user.js, answering Yes when Windows asks if you're sure you want to change the extension. Right-click the user.js file and select Edit to open it in Notepad. Then add the following line to the end of the file:
Save the file and quit Notepad.
Now launch Thunderbird and start a new message. Enter the To: address as usual, then click in the space below the word To: to see a list of other available headers. Select X-Message-Flag from the list and enter your chosen warning message. Add the subject, compose your message, and send. A recipient who uses Outlook will see an official-looking warning, though most other e-mail clients won't show it.
You can also force Outlook to use red text for your message's line in the message list. All you need to do is add a date and time to the Reply-By item in the header and make it a time that has already passed.
To do so, pull down the same list below Thunderbird's To: field and add your X-Message-Flag warning, then pull down the list on the next line and choose Reply-By. The date/ time must include your time-zone adjustment in the following format: Apr 1 2005 12:00 -0800. The time information will be appended to the warning message you created.
Now that you've seen how easy it is to make Outlook display an arbitrary warning message, you know to take such warnings with a grain of salt.