Not able to configure "Send mail as:" On Gmail

Hello, 

I want to configured Gmail (using their "Send mail as:" feature) to send mails through my business Office365 Outlook account, , but when i try to connect it I get a Authentication failed message from the Office 365 server with an error message like this:

Authentication failed. Please check your username/password.
Server returned error: "334 VXNlcm5hbWU6 334 UGFzc3dvcmQ6 535 5.7.3 Authentication unsuccessful [SN6PR01CA0011.prod.exchangelabs.com] , code: 535"

I can sign in to Outlook Web App without problems using the same credentials.

Below is the screenshot for the same 

Can some help me to fix this issue