In your Google Sheet that contains your contact list with email address and other merge fields, go to Add-ons menu, and click "Small Outbox"
Choose "Format Sheet"
Add-ons will add two add additional sheets called "Gmail Drafts" and "Options"
3. Create Personalized Email Template
Open Gmail
Create a draft email without recipient
Put in all contents required including inline images and attachments
Now put in merge fields by placing your contact sheet column name. For example, if you have a column in Contact sheet called "Membership", simply put {{Membership}} in your email and when mail merge happens, Small Outbox will replace that with the actual value on the data row
Note that the merge fields are case sensitive
4. Make Sure Contact Sheet
Make sure the "Contact List Sheet Name" in the "Value" column, it populates the contact sheet name you need to do mail merge. In this example, it is Sheet1
5. Copy Email Draft ID
Navigate to Gmail Drafts sheet
Find the Gmail Draft you want to perform mail merge
In this example, we choose the first one
Copy the value from ID column
Paste it into the "Email Draft ID" in "Value" column
6. Make Sure Enough Daily Quota
There is a cell called "Daily Quota", which indicates how many daily email recipients you can send
In this example, there are still 1500 email recipients that can be sent
If this value becomes 0 and you perform mail merge, you will receive error message
7. Verify Status
When sending, the "Status" field in "Options" sheet will change from "Pending" to "Sending"
The whole job may take a bit time as it is capable to sending 1500 emails without breaking Script Runtime quota
When the whole job is completed, the "Status" field in "Options" sheet will change from "Pending" to "Completed"
Also in the contact sheet, there will be two extra columns inserted at the beginning to indicate the time sent and status of it
Currently the status will only show "SENT" but later there will be a bounce checking that will indicate if the email was "BOUNCED"