Nodemailer v4.5.0 includes a new configuration option normalizeHeaderKey. It's is an optional method that is applied to every email header key when generating the RFC822 formatted email message. So instead of "X-Some-Key: abc" you could have "X-somE-keY: abc" in the message header. Mostly useful for vendors that expect some header key to follow a specific casing rule and the default normalization that Nodemailer applies is in conflict with that.