Send Mail Features
Send Mail on Behalf of
If you want to send emails on behalf of another email account you need to first configure it from Send mail as with in the Accounts tab of your Gmail settings. This is ideal if you want to send emails on-behalf of your boss or an other user within your organization.
You can also send email to non-Gmail accounts such as those by Microsoft, Yahoo!, SendGrid or any other email server which supports SMTP. Just follow these steps:
Once the new account is added. Refresh the add-on and under 'Show Email Settings' you should find the option to choose from which email account you want to send the emails from.
Send emails over SMTP
Available only from the Google Docs addon
The Send mail as feature described above, will not allow you to send emails over smtp for your own account or other accounts with in your G Suite domain. Therefore if you want to use a 3rd party email provider such as SendGrid to send emails over SMTP with your gmail account you need to set the provider's smtp settings from Google Docs->Add-ons->Mail Merge->SMTP Settings.