Enter Button With Email Textbox

i have already set panels for each table in my page. i am using validation summary's message box to show validation messages. On my login page i have a email txtbox and a password txtbox. the problem is that if you type "fff" in the emailtxtbox and leave password blank then hit enter button on email txtbox, the error message doesn't show. But if you type in "fff@fff.com" and leave password txtbox blank and hit enter button on email txtbox then the message shows.. how to fix this?
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