I would suggest that you create a domain debug log for the "lirex.bg" domain. By enabling the domain debug log, it will allow you to see in detail that smtp conversation when your Ironport appliances tries to hand the mail off to the lirex.bg domain.
To create the domain debug log, on the GUI interface, go to "System Administration > Log Subscriptions". click on new , select the domain debug log and configure it to record for "lirex.bg" and 10 sessions.
From the sample mail log entry, I looks like the formatting of either the "mail-from" or "rcpt-to" or possibly the header(from,to) is in a non RFC 8222 format that the other side doesn't like.
The domain debug log will show how much data has been transferred and what the other side doesn't like.
What does it mean:
deliver mail from', ' to . Please try', 'later.'])  Mon Apr 7 12:18:06 2008 Info: Delayed: DCID 1637258 MID 2659459 to RID 13 - 4.1.0 - Unknown address error ('451', ['184.108.40.206 is not yet authorized to deliver mail from', ' to . Please try later.']) 
I asked because we cannot send or recieve mails with domain name lirex.bg, and when I checked maillogs I found this. What can be the problem?
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...