mail/internal/mail/message.go
func (m *Message) SetBodyWriter(contentType string, f func(io.Writer) error, settings ...PartSetting) {
	m.parts = []*part{m.newPart(contentType, f, settings)}
}
func (m *Message) SetBodyWriter(contentType string, f func(io.Writer) error, settings ...PartSetting) {
	m.parts = []*part{m.newPart(contentType, f, settings)}
}
// SetBodyWriter sets the body of the message. It can be useful with the
// text/template or html/template packages.