I guess it depends what kind of mail you're sending? I send & receive a lot of mail, but I can't imagine anyone has ever marked a mail from me as spam (except perhaps by clicking the wrong button), since it's just normal conversational personal & business mail. I never did any kind of deliberate 'warming up', I just set up the server and then started sending mail. In the first few months people would occasionally see mail from me go to their spam folder, but that hasn't happened in many years.