Yahoo CEO Marissa Mayer in December 2012, unveiled a redesign of Yahoo Mail aimed at making the service more sleek and user-friendly but Yahoo Mail if suffering from security vulnerabilities and many customers have been under assault from hackers.

Yahoo's hacking problem are occurring due to Yahoo's decision to not implement two-factor authentication -- a feature that sends additional one-time passwords to users' cell phones to make their online accounts more difficult to hack. Gmail and Outlook.com currently offer two-factor authentication.

A British telecom company says it will no longer make Yahoo Mail the default email service for its 6 million customers due to concerns their accounts are vulnerable to getting hacked and they decided to go for BT Mail.

So, yahoo is currently losing a fraction of it's customer base of about 280 million people worldwide.Y ahoo is now the third-largest email provider after Google’s Gmail and Microsoft’s Outlook.com. Losing so many customers might be troublesome for Yahoo as it highly depends of the revenues generated by advertisers in the email.