Undelivered mail returned to sender

[RENIL]

This is the mail system at host mail.xxxxxx.com.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                   The mail system

<ixxx@outlook.com>: host xx.xx.xx.xx[xx.xx.xx.xx] said: 550-Please turn on
    SMTP Authentication in your mail client, or login to the 550-IMAP/POP3
    server before sending your message.  (mail.safetyoman.com)
    550-[5.36.xxx.xxx]:43939 is not permitted to relay through this server
    without 550 authentication. (in reply to RCPT TO command)

I am new in Zimbra and the above problem not happening usually..
Sometimes i can mail to outside zimbra and some times it is not working..
Please  any one help me..
Thanks.