Nodemailer v4+ is licensed under the MIT license.
Version 3 of Nodemailer used the European Union Public License 1.1 which is a copyleft license. You can read about the reasoning behind using EUPL for Nodemailer from the Nodemailer blog. After a two months long experiment I decided to cancel all active subscriptions of Nodemailer and changed the license back to MIT.
Keeping a lower level infrastructure module under a restrictive license has turned out to cause more trouble than gains – I myself had to use older versions of Nodemailer for some client work to avoid any licensing conflicts. There weren’t too many paying or interested clients either and most users did not seem to care about the license at all, they still used the copyleft licensed Nodemailer in their commercial products and I did not have any means to enforce the license terms.
If you really like Nodemailer or your business benefits from it financially then I would really appreciate a small donation. You can either use Bitcoin or PayPal for donations.