There are few agencies that U.S. citizens fear and loathe more than the IRS. And of course, this is reflected in the copious amounts of spam that solicits money on behalf of federal agency. Recipients will typically receive an official looking message, allegedly from the IRS, notifying them that there was a problem with their tax return. Or that they need bank account numbers and other financial information in order to make a deposit for the return.

Spammers will generally unleash phony messages around tax time, just when everyone is really nervous, experts say. "Here we have a brand, which is a government agency, that most people don't want to have any complications with," said Jon Orbeton, strategic product manager for Ironport. "They can wield so much power against you if you're not paying your taxes."