In the last few weeks a new type of spam has been on the rise: Meeting requests that are sent using the calendar system in Microsoft Outlook and Google's online calendar tool. I call it "c-spam."

I've been hit with three of these c-spam messages in the last week (I use Outlook) and they present a real challenge over regular spam for a variety of reasons:

First, they appear to bypass most spam filters, which generally only scan standard messages and skip meeting requests.

Second, even if you simply delete the message without accepting the request, the meeting request still shows up in Outlook, typically as a blocked-out, all-day meeting. (I didn't realize this until a pop-up announced I needed to get ready for some URGENT BUSINESS ASSISTANCE the following day. It's two spams for the price of one.)

Finally, you can always "decline" the meeting invitation, which would remove it from your calendar, but this works as a perfect means of letting the spammer know that the email account they're spamming is not only active, but you have Outlook installed and are diligent enough to have read the message you were sent and use the calendar tool. Clicking "decline" is the perfect way of inviting lots more spam of every kind into your inbox.

Aside from waiting for spam filter tools and Microsoft to catch up with this problem, there's not a whole lot you can do about meeting spam. Microsoft is clearly already aware of the issue, recommending that users turn off "automatically [sic] acceptance of meeting requests" as part of its guide to avoiding spam. (You probably needn't actually follow the steps to do this, as that option is turned off by default.)

I have little doubt that c-spam will be a growing trend among spammers. Best thing you can do now is be aware of the problem, stay diligent, and be liberal with the delete key.