I added some custom field email_bcc on mail template model and also on the mail_mail model to accept the email_bcc data. And also did the appropriate changes on send_mail, generate_email functions to add the email_bcc field.

While I am sending email its success and email bcc is added to the email. Problem is the recipients under Bcc is shown to all, while sending from odoo. 

I set the outgoing mail server with a Gmail account. So can from Odoo, we need to do anything extra or is it related to the mail server?