e371f003_liverability___Act_On_Software__Are_You_a_Legitimate_Mailer_
Spammers try to hide malware in invisible elements. [Are You a Legitimate Mailer?] 
Do not use invisible web bugs to track your emails. If you must track your emails and whether they’re read, use visible graphics.
Best Practices in Email Deliverability - Act-On Software
Are You a Legitimate Mailer?