What does this error message mean?

The message returned by the recipients server, indicates that the message which you sent has traces of a spam mail. This may be due to some long or suspicious links or attachments in the mail.

Many service providers filter email messages containing a link, attachment, or pattern caught by their filters as spam or a possible virus.

How do I solve this issue?

Try to cut down the number of links or any long links and resend the mail. This should be solved by removing the suspicious content.