"Compared to five years ago, more scams [and] illegal, fraudulent, or spammy messages today come from someone you know," Google security engineer Mike Hearn said in a blog post.
"Although spam filters have become very powerful -- in Gmail, less than 1 percent of spam e-mails make it into an in-box -- these unwanted messages are much more likely to make it through if they come from someone you've been in contact with before.
"As a result, in 2010 spammers started changing their tactics -- and we saw a large increase in fraudulent mail sent from Google Accounts," he wrote.
Spammers are increasingly attempting to break into legitimate accounts and send mail to those accounts' contacts, CNET reported Tuesday.
"We've seen a single attacker using stolen passwords to attempt to break into a million different Google accounts every single day, for weeks at a time," Hearn wrote. "A different gang attempted sign-ins at a rate of more than 100 accounts per second."
Google has recommended users take advantage of the company's two-step verification and recovery options to secure accounts.