Sorry, a policy is in place that prevents your message from being sent.
The intended recipient of your message didn't receive it, because the message doesn't meet the mail policy requirements for your organization or for the recipient's organization. For example, the message may contain words that the organization considers objectionable, or the organization may limit who can exchange mail (for example, a school might allow its students to exchange mail with the faculty and other students, but not with people outside of the school).
If you've successfully sent mail to this recipient in the past, check the message to ensure that it doesn't include protected information such as credit card numbers or words that the organization might consider objectionable. Many organizations also limit the size and type of permitted file attachments.