Outbound mails stuck in Backend server queue

I have two Backend and one Frontend Exchange 2003 servers, suddendly all outbound messages sit in the Backend servers and there's nothing on the Frontend server.  I've recreated the SMTP connectors and rebooted the servers.  The IP address in the Virtual SMTP properties is the IP address of the Exchange server and no "All Unassigned". In event viewer on both Front end and Backend servers I have many "504 Need to authenticate first"  "Xexch50 10842" errors
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lanreoConnect With a Mentor Author Commented:
For testing purposes, I enabled the frontend server to relay only for the two backend servers, as soon as I stopped the Virtual SMTP on the frontend, all the outbound emails that were stuck in the backend queues got delivered.  I started the Virtual SMTP and outbound stopped working again.  Please help, I need to fix quickly
Praveen BalanSolution ArchitectCommented:
What happens when you do the telnet from backend to friend end server.

>telnet friendendserver 25

see if the telnet session opens and shows the banner of FE server. If not, you have some issues with your SMTP service in FE server.

try to send email using SMTP commands if the telnet works fine, post your updates
lanreoAuthor Commented:
Thanks for you prompt response...I got pulled into an emergency meeting:
The tenet worked but when I tried to send email with smtp command, I got the error:
"550 5.7.1 Unable to relay for xyz@yahoo.com."  I tried hotmail.com and got the same error
lanreoAuthor Commented:
No response from anyone else
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