It's interesting that you would mention that Nodemailer is not appropriate for mass mail due to the callback structure - does it make sense to write off mass mail because of this? I'm not convinced it does. What if you could specify a write stream that the result of each send could be streamed to? Or maybe do callbacks with batch results? Throwing away mass mailing in the biggest mail library available for Node seems sort of silly to me.
Of course, a big thanks is in order - Nodemailer has been a great system to work with and I'm glad it's still getting the love it deserves. Keep up the great work Andris.
What I actually mean is that you probably should not try to build your own SendGrid on top of Nodemailer. You might try and it might even work but you probably should not.
If you want to send 10 000 e-mails at once, then it works just fine. If you want to send 10 000 000 messages from a for loop then it probably breaks.
Of course, a big thanks is in order - Nodemailer has been a great system to work with and I'm glad it's still getting the love it deserves. Keep up the great work Andris.