e371f003_liverability___Act_On_Software__Are_You_a_Legitimate_Mailer_
Invisible text is often identified as a sign of spam. [Are You a Legitimate Mailer?] 
Do not use invisible text within emails. Make sure your text colors and sizes are distinct enough and large enough to read.
Best Practices in Email Deliverability - Act-On Software
Are You a Legitimate Mailer?