mass emailing with Google Apps scripts and validating email lists

One of the ways in which we can send "Mail merge" equivalent customized emails to a large number of users would be using the sample script at
https://developers.google.com/apps-script/samples/automations/employee-certificate

- this seems to be a much better solution than GMass (free option) which I had earlier posted about, but requires some coding. 

This works quite well if all the emails are pre-verified. Our non-profit accounts have ~1500 emails per day as reported by

var emailQuotaRemaining = MailApp.getRemainingDailyQuota();

Logger.log("Remaining email quota: " + emailQuotaRemaining);

 
But the important thing is that the list of emails must be accurate. If multiple emails bounce, the whole process will get derailed, and the user sending the emails will get suspended. We should probably also go slowly so that rate limiting does not kick in. 

One of the good ways to ensure valid emails might be if all these emails in the list are obtained using the users filling up some google form which requires google sign-in. 

A youtube video for validating email lists uses SMTP check being done with api from

https://mailboxlayer.com/product

An alternative might be https://www.myfreshtools.com/misc/bulk-email-validator-tool

More ways to do smtp check - 

https://stackoverflow.com/questions/565504/how-to-check-if-an-email-address-exists-without-sending-an-email

We could also add the email ids to a google group - that would also validate their email to some extent, with at least hotmail, yahoo and gmail ids being caught if invalid. 

Edit: Some interesting reading about spam filters and "warming up" - 

https://www.gmass.co/blog/free-email-warm-up-tool/