Do you/did you have feedback loop accounts setup for all of them? Yahoo, for example, sends a notification to sys@ (or whatever email you tell them to) when someone marks one of our emails as spam or when it thinks we're sending bulk email. They also have sender support. They did want me to set up something where we used the same connection to send email to them. I forgot

My "blacklisted" email company is Zoho because they consistently greylisted and blocked our correspondence with clients and have no one to contact to resolve email issues for senders.