Email Contains potential security issue

Hi community,

I get the following error 90% of the time when I'm trying to send an email to someone.

You can no longer send emails to this particular email address for the following reason: smtp;552 5.7.0 This message was blocked because its content presents a potential security issue. Please visit https://support.google.com/mail/?p=BlockedMessage to review our message content and attachment content guidelines. o9si10191575edr.292 - gsmtp

Any advice of how can I fix this?