Mail Tranport with authentication

[shivkumar.dvs]

I followed the split domain document in wiki and configured to forward all the mails of my domain to external smtp server hosted at ISP(as currently I am testing zimbra) and also created outbound relay to my ISP with authentication, so now when I send a mail to other domains it sends via relay using SMTP authentication , but when I send a message to a user at my domain currently at my ISP it forwarding the message to the relay server but it is asking for authentication, I used zmprov tool to catchall message from my domain and relay to my ISP. I did not find any option to set authentication there.

Could you please help me out on this, I would like to know is there any way to authenticate using zmprov mailtransport attribute.

Regards