Content filters work on the content (or body) of email messages. While not intended to interfere with legitimate senders, mistakes can occur.
Content filters work by scanning the text of an email for signs of spam or fraud. Historically, they were based on trigger keywords such as free offer. These they are much more advanced so ignore any advice they tells you to avoid using certain words or phrases. If your email does get caught by a content filter it is usually because it inadvertently shares a specific trait commonly associated with spam or fraud emails.
The good news is that it is usually fairly easy to make legitimate email not look like spam. Get in touch if you need help understanding content filtering.