Speaking at TechCrunch Disrupt conference in San Francisco, Mayer said the figure excludes traffic from its blogging and social networking site Tumblr, which it acquired for $1.1bn (£925m) in May 2013.
The search engine also captured more US users than Google in July 2013, for the first time in two years.
"It's just email. It doesn't have video chat and a lot of these other things, which we may experiment with but I think can ultimately get in the way in terms of using mail every single day and wanting to be really fast and efficient with it," Mayer said.
"The products have to be good.
"Otherwise, users don't come and actually use them."
Despite the rise in traffic, Yahoo still lags behind Google and Facebook in generating online advertising revenue.
Yahoo also said that 350 million of its active users are on mobile, with the remainder coming from the home page, mail, and search.