SO16 Settings are not working

Dear Experts,

I am trying to send mail to the external mail id mentioned in SU01 tcode. For that, I have set up a 'Send to Home Addresses of users' option under 'Mail Sy. grp' tab in SO16. But these settings are not working. SAP is not sending mail to the id's and it is giving an error.

If I create the Send mail step in workflow it is sending mail successfully.

Please help....