Key Difference: Yahoo! Mail is a free email service offered by Yahoo! On the other hand, Gmail is also a free email service provided by Google.

Both, Yahoo! Mail and Gmail are free email services provided by different companies. However, there are alot of similarities in the features and functions offered by them.

Yahoo! Mail was launched in 1997 and was the third largest web-based email service uptil December 2012. Later, in 2008, Yahoo! Mail started offering unlimited storage space to its users. This strategy was used to gain the market back which had strict competition in the free email provider segment. As of now, there are two versions of Yahoo! Mail available online; the Classic Yahoo Mail interface with which the company started and the Ajax interface version launched in 2005. However, many alterations have been made to the Ajax version. In October 2010, Yahoo! released Beta version of Yahoo! Mail with improved performance and Facebook integration. In 2012, their current WebMail interface was introduced.

Likewise, Gmail is also a free email service provider, started on April 1, 2004. It was initially started with invitation-only beta version and became available to the public on February 7, 2007; still in beta version at that time. On July 7, 2009, the service got upgraded from beta status to a complete Google Apps suite. Initially, Gmail started offering 1GB of storage per user. Currently, the storage limit is 15GB per user; combined with Google+ Pictures and Google Drive service. However, more storage can be allotted to users on a rental basis.

Yahoo allows a user to send emails with attachments up to 25MB and up to 100MB via the built-in 'Attach Large Files' app. Also, Yahoo! allows sending emails with attachments up to 2GB to its Yahoo! Mail Plus accounts. Whereas, Gmail allows a user to attach files up to 25MB; although, larger files up to 1GB can be sent by using another service known as Google Drive. 

In August 2010, Google released a plugin for voice calls and later for video calls in Gmail. These services are not available with Yahoo! Mail; however the same services can be used from Yahoo! Messenger.

Both, Yahoo and Google, as internet companies have a wide range of services to use. However, the differences in their email services are not much, still the differences are noticeable in terms of attachment size and storage.

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