The new mail app has many improvements along with new design
With the announcement of OS X Yosemite Apple has updated the Mail app with some nice features with minimal looks. The Mail app now looks alot like the one we see on iPad runing iOS 7. There is a new feature called MailDrop which will allow you to easily send large files via your mail app.
Most of the people cannot send large files due to restriction from the mail clients so what MailDrop does is that it uploads your files to Apple's servers creating a secure link and the files are sent without any problem. You can send files upto 5Gb, the mac users will get the file as it is while the other users will get the download link.
Mail app also features Markup which allows you to sketch over the images or add text to them. It will be alot more easier to make changes to the images you want to send and you can even add text along with bubbles and shapes. So the Mail App is fast, easy and reliable with even more productivity now.