Nodemailer can be purchased from Kreata OÜ. License fee (excluding VAT) for a 1 year license is $820.-.
Nodemailer is available under a more restrictive license without a fee, so you are free to test Nodemailer before actually buying anything.
Nodemailer subscription for the commercial license is sold on Gumroad. Contact sales [email protected] for any sales related questions. For support related issues contact [email protected].