MAAWG has published their position statement on email appending. It’s pretty explicit in it’s condemnation of the practice.
It is the position of MAAWG that email appending is an abusive practice. Sending email to someone who did not explicitly give informed consent for his or her email address to be used in this way is never acceptable. It will result in complaints, which only further illustrates how much end users find this practice abusive. It will result in delivery issues, largely as a result of those complaints. Legitimate marketers do not engage in email appending.
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MAAWG isn’t mincing words here. They do not support sending mail without permission. They do not believe that matching an email address with a customer record constitutes permission to mail to that customer.