Microsoft (NASDAQ:MSFT) is using its Scroogled.com domain to take jabs at Google (NASDAQ:GOOGL) again. Google's Gmail recently began serving ads in its revamped inbox feed, and Microsoft's letting Big G have it.

"Google spams your inbox with ads that look like real emails," Scroogled proclaims, complete with a video and slideshow that illustrate how Google is placing ads at the top of the new Promotions tab of the updated Gmail platform.

To be fair, it is a bit jarring. The ads are clearly identified at the top of the inbox -- set apart from the incoming email with its lightly colored background -- but it's something that will take some getting used to. Unlike the emails that can be checked off on the left and deleted, the ads get nixed on the right side of the feed. Instinctively clicking on the left will merely open up the ad. The sponsored spots also can't be labeled as spam, even though that's how many users may feel about the situation. 

Anyone wondering why Gmail was presorting incoming email earlier this summer -- and branding it as a feature -- now has every right to be cynical. Microsoft obviously would prefer that Gmail users get fed up with Google's platform and set up camp on its own Outlook.com.

This isn't the first time the software giant has aimed at the search leader through the Scroogled website. It was launched ahead of last year's holiday shopping season to point out how Google's shopping hub is populated with paid product listings. Scroogled turned its attention to Gmail earlier this year, pointing out that Google sells targeted ads based on email contents. Google has countered the allegations, but Microsoft's the one scoring the style points as Google haggles over substance.  

Yahoo! (NASDAQ:YHOO) is the third of the major players in email, but you won't see Microsoft trying to bruise the folks who bleed purple. Yahoo! isn't following Google's monetization practices, but -- more importantly -- Microsoft and Yahoo! are search partners. Microsoft isn't going to do anything that will damage the relationship where Yahoo! outsources its paid search business through Microsoft. 

There also is more to gain by taking Google down a few pegs. Google isn't just the leader in search. Google's Android is the top dog in smartphones and tablets, where Microsoft's commanding a mere 4% share of the market. Getting consumers to turn on Google would be beneficial for Microsoft's well-being beyond free email.

Microsoft's been on the other end of an attack ad campaign, and it worked. The "I'm a Mac, I'm a PC" Apple (NASDAQ:AAPL) ads killed Vista. The market accepts low blows when it's an underdog doing it, and that's what Microsoft is these days. No one flinched earlier this year when its Lumia ads made fun of Apple and Samsung smartphone owners. It also won't be a surprise the day Apple begins attacking Google the way it did Microsoft several years ago, even if "I'm an iOS, I'm an Android" doesn't have the same ring to it.

Microsoft will keep on swinging, and why not? It's the rare reward for being the underdog. 

Longtime Fool contributor Rick Munarriz has no position in any stocks mentioned. The Motley Fool recommends Apple, Google, and Yahoo! and owns shares of Apple, Google, and Microsoft. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.