If you are sending a lot of emails, you really need to look at using a bulk email provider. They are built to handle a large volume of outgoing email, and will help ensure your emails are delivered.
I've used SendGrid and Mailgun successfully from Email Template in the past, but other companies offer similar services. Gmail isn't really meant for this.
I know this isn't really the answer you are looking for, but it's the solution I use, and it works.