email validation in input text box.

I have 2 or more emails in my email input text box..its separated by commas.. I will check the following conditions..
1.checking email input text is null or not null..

if the value is null,the error "please enter the email"(it show in red colour,) will show under the email text box..

2. If the text box have the value it will check the all emails have valid or not(checking each mail is valid or not valid)..if it is not valid it throws the error under text box..