mail/internal/mail/message.go
func (m *Message) SetDateHeader(field string, date time.Time) {
	m.header[field] = []string{m.FormatDate(date)}
}
func (m *Message) SetDateHeader(field string, date time.Time) {
	m.header[field] = []string{m.FormatDate(date)}
}
// SetDateHeader sets a date to the given header field.