delete all email from Samsung galaxy s3 at one time?
Go to Settings => Storage => Used Space => Applications

Find your email app in the list and select it. For example mine is Gmail.

Click on "Clear data". You will get a warning saying you will have to set up your account again. It is your choice if this is what you want to do.

Once you have done this, go back into your email app as you normally would to check your emails and enter your account details. In the Settings of your email app you should set things up not to keep so much email. So for example in Gmail you can reduce "Days of mail to sync" from the default 30 to for example 5.