
Are you using outlook.com (including hotmail, live, msn) or a custom domain?
If the recipient's mail filter is treating mail from outlook.com as junk mail, only they can fix it.
If you are using a custom domain, you need to have SPF, DKIM, and DMARC records in your DNS. If outlook.com is the custom domain host, you cannot add DKIM records and would need to move it to a new host.