unable to do scan to email

Hi,

We have a Dell Color MFP S2825cdn and I tried setting up the scan to email feature. I tried the following settings:

SMTP server: smtp.gmail.com

Auth: tried invalid, SMTP(AUTO), SMTP(plain), SMTP(Login): all failed

port#: 25, 465, 587

it just shows unable to connect. Though this smtp server and the login I setup is the same I used on a different printer, so I'm pretty sure this Gmail smtp is working. 

Thanks

Jeff