Sending emails on behalf of someone else
Do you own an other email address and would like to send your mail merge through that other email account? and potentially even with a different reply-to address?
If so, you need to configure your gmail account to be able to send from an other email address. To do so please follow the instruction here.
Once you've finished setting it up refresh your Google Document Template to reload the Mail Merge add-on. When you expand the 'Show Email Settings' you will see a new drop down entitled 'From Email Account'. Just choose the account you wish to send emails from.