My Mail in Yosemite doesn't work!

I am having problem with my Mail in Yosemite OS, hotmail is fine, only gmail has problem. I can receive emails, but i can send mails with gmail. Every time I try to send an email with my gmail in Mail, it tells that "Cannot send message using the server smtp.gmail.com:(my gmail user name)." it also says "The gmail SMTP connection to server smtp.gmail.com failed."

What should I do? I need to send tons of emails each day, and now i have to write my emails on my iMac, then open it in my iPhone 6 to send emails!