This article describes how to send emails from Google Docs over SMTP using a third-party provider (such as SendGrid) to overcome the email quota limits imposed by Gmail.
Click on Extensions > Mail Merge > SMTP Settings.
Select Yes from the first drop-down menu, and then enter the SMTP settings for your chosen email provider.
SMTP settings for some popular services can be found in this article.
Click on Save to apply the changes.
NOTE: If you are using Gmail SMTP, you need to enable Less Secure App Access.