mail/internal/mail/message.go
func SetPartEncoding(e Encoding) PartSetting {
	return PartSetting(func(p *part) {
		p.encoding = e
	})
}
func SetPartEncoding(e Encoding) PartSetting {
	return PartSetting(func(p *part) {
		p.encoding = e
	})
}
// SetPartEncoding sets the encoding of the part added to the message. By
// default, parts use the same encoding than the message.