This spam check report is actually just a warning which says that some email server could treat your emails as spam because of the hidden text. Text which is hidden in the newsletter is the email preview line. That line allows some email clients to show the extract of an email in the list of emails, which it shows.
If you want to avoid this, you should remove Email preview content from your newsletter
I am afraid that there is nothing else we can do about this. Email preview line is something that is used regularly in emails. The majority of email servers will allow that to be used and won't complain, but there are some servers, which are not configured properly, which will mark emails containing that text as spam.
As said, if you want to be on the safe side, don't use Email preview and all should be good
