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Yahoo's free email service is becoming a bit more like Google's Gmail as part of its second makeover in less than a year.

The similarities to Gmail probably aren't coincidental. Yahoo CEO Marissa Mayer helped design some of Gmail's features while she was a top executive at Google Inc. Since its debut nearly a decade ago, Gmail has grown into the world's most popular email service.

Yahoo's redesigned email unveiled Tuesday includes a Gmail-like tool that will thread together emails related to specific topics so they appear as a succession of messages. The "conversation view" has become a widely used email feature since Gmail helped popularize the concept after it embraced the format in 2004.

Users can turn off Yahoo's new conversational tool if they want.

Another new feature will enable Yahoo's email users to decorate their inboxes with a selection of scenic pictures plucked from the company's photo-sharing service, Flickr. Gmail has been allowing its users to spruce up their inboxes with various themes for years.

When Yahoo's email users choose a picture as their backdrop, the same look will automatically appear on the mobile email applications that the company is modifying as part of the redesign. The updated apps are for Android devices, Apple Inc.'s iPhone and iPad and tablets running on Microsoft Corp.'s Windows 8 operating system.

In another change, Yahoo is now promising each email account a maximum of one terabyte, or about 1,000 gigabytes, of storage. The Sunnyvale, Calif., company says that amount should be enough to cover the storage needs of its average email user for about 6,000 years. Yahoo Inc. had previously promised its email users that they would never run out of storage, but it hadn't established a specific limit.

Gmail vastly expanded the capacity of email boxes in 2004 when it rolled out its service with a limit of one gigabyte per account. At the time, industry-leading email services run by Yahoo and Microsoft Corp. were limiting storage on their free accounts to 25 megabytes or even less.

Yahoo's terabyte limit now dwarves Gmail, which has a per-account limit of 15 gigabytes that also includes material kept on Google's Drive and Photo Plus services.

Since defecting from Google 15 months ago, Mayer has been revamping many of Yahoo's services in an attempt to attract more Web surfers and bring in more revenue from ads. Yahoo's ad sales remain lackluster at a time Google and Facebook Inc. are enjoying strong growth, but Mayer says the number of monthly visitors to the company's services has increased by 20 percent to 800 million people since her arrival.

Yahoo's last major overhaul of its email service occurred in December. The company now has about 289 million monthly users worldwide, second only to Gmail at 304 million, according to the most recent data from the research firm comScore Inc.