e371f003_liverability___Act_On_Software__Are_You_a_Legitimate_Mailer_
Emails with missing mime sections, invalid or missing message-ids, invalid or missing date headers, subject or other headers with unescaped Unicode (and so on), are frequently spam. [Are You a Legitimate Mailer?] 
Use email composition and mailing tools that work correctly. Well-constructed emails (technically correct) can be readily identified as not-spam.
Best Practices in Email Deliverability - Act-On Software
Are You a Legitimate Mailer?