e371f003_liverability___Act_On_Software__Are_You_a_Legitimate_Mailer_
If a bulk mailer’s product’s feature list includes ‘stealth sending’ or similar terms, all mail sent by that program will be treated as spam. [Are You a Legitimate Mailer?] 
Don’t use ‘bulk-mailing’ tools used by spammers or advertised through spam.
Best Practices in Email Deliverability - Act-On Software
Are You a Legitimate Mailer?