551 This mail server requires authentication before....

Dear Guru's,

I've made most of the settings in SAP in order to send automatic P.O. to external e-mail but now the system gives this error and doesn't send my P.O. via e-mail.

As far as I understand it's about SMTP server doesn't trust SAP server. In anyway SMTP server needs to know my SAP server but I couldn't find out how. Is there any place that I can enter username & password in SAP. Or the settings should be done in SMTP server about rely?

Please kindly advice how to proceed.

Thanks in advance,

Best regards...