Shameless plug, if your email addresses and customer data are in a database, you can use SeekWell (www.seekwell.io) to pull the data into Sheets then run Amit's excellent product.
Thank you so much for making this Amit! This script is really effective when I want my email to seem more personalized. I get much higher opens and click-thru rates with your script compared to bulk emails through ESPs like MailChimp, Constant Contact etc...
For example: I used your method to send out 150 emails this week. 83 of the 150 people CONVERTED into my facebook group members. I would not have had a 55% conversion rate without using your script to send through Gmail. Thank you sir :)