Marketers send these emails because they are free and they see some engagement. There is no metric to calculate whether their customers are upset receiving thes

Marketers send these emails because they are free and they see some engagement. There is no metric to calculate whether their customers are upset receiving these mails. Whether it‘s an effective method is not clear. But it is not intelligently used (only send follow ups if e.g. not created task in a todo app) by almost anyone.