Port 587 for smtp

Some of my users are trying to access their company email through clients like macmail and outlook outside the firewall at home. They are able to receive great but cannot send. I have worked out the issues with most providers but Verizon is the pain. They have port 25 blocked and recommend using 587. This still does not work. How do I open port 587 in my firewall so that I can configure my smtp to use that for webmail. I have a Fortigate 200a running mr7 patch 4. Thanks.