Ads
If all ads are removed from Yahoo! Email services completely, you'll save more money than you make with ads.
Why? Because ads slow systems down, they could be (and often are) scams or contain malicious code that steals personal information. This will, I'm sure cost Yahoo!'s parent company billions yearly in upgrades to cater for different ads, not only to have on our screens, invading our privacy, but to better protect your own servers from malicious code.
Remove all ads and reduce the world's depression, anxiety levels will drop, efficiency will grow, systems require less expensive maintenance, Yahoo! Saves at least treble what it makes from ad revenue.
Or just be Uber greedy and hostile towards users by implementing the evil malware known as Unity Ads. You'll reduce your active users, ad revenue decreases, Yahoo! Goes Bust.