"It's not something everyone knows about, and most people don't approve of it," Stefan Weitz, Microsoft's director of online services, said in a phone interview.
A Google spokeswoman said ads allow the company to offer its services free of charge.
"No humans read your email or Google account information in order to show you advertisements or related information. An automated algorithm — similar to that used for features like priority inbox or spam filtering — determines which ads are shown," the spokeswoman said.
But Weitz argued that most people find the practice invasive.
"When you're engaged in a conversation with someone on email, you assume that it's a private conversation." he said. "This kind of eavesdropping makes people uncomfortable."
As part of the campaign, Microsoft launched a petition on Scroogled.com urging Google to stop selling ads based on the contents of its users' emails.
Microsoft also scans the content of Outlook emails to protect against spam and viruses, but it does not use the information for targeted ads. Like many ad networks, however, Microsoft displays ads based on the websites that users visit.