Unable to send mails from external mails to zimbra mail

[segun]

Please am new to zimbra, and so far i was able to setup my zimbra server to send mails to external mails like yahoo and gmail from my zimbra server but am unable to send from those external mails to my zimbra server.
Below is the failure notice am receiving back from my yahoomail


Sorry, we were unable to deliver your message to the following address.

<segun.ajayi@etranzact.com>:
Remote host said: 550 5.7.1 Unable to relay for segun.ajayi@etranzact.com [RCPT_TO]
Received: from [98.139.91.70] by nm11.bullet.mail.sp2.yahoo.com with NNFMP; 15 Sep 2011 09:02:54 -0000
Received: from [98.139.91.32] by tm10.bullet.mail.sp2.yahoo.com with NNFMP; 15 Sep 2011 09:02:54 -0000
Received: from [127.0.0.1] by omp1032.mail.sp2.yahoo.com with NNFMP; 15 Sep 2011 09:02:54 -0000
X-Yahoo-Newman-Property: ymail-3
Please can someone help me out.

[hanuman.mumbai1947]

I Think your IP Is Blacklisted By Yahoo & Gmail