After the normal upgrade I'm unable to send/receive any email from my Mail server.

Can someone help me out on this?

Here's the error I got:
method:	[unknown]
msg:	try again: Unable to connect to the MTA
code:	mail.TRY_AGAIN
detail:	soap:Receiver
trace:	qtp1680275375-87:http://[SERVER's IP]:81/service/soap/SendMsgRequest:1350842833307:bed7f621cbf73f2f
I've double checked configurations on Hosts/Zimbra Admin and everything seems to be properly configured.

Looking forward to hearing from you.