550 5.7.1 Unable to relay

220 pts.
Microsoft Exchange 2003
HI, we have a issue with our mails as some of our mails are bouncing back with "550 5.7.1 Unable to relay " error randomly.I have checked all our relay setting and everyting is fine from our side. But it happens very frequently for different domains and it will get resolved by itself after few hours. It is very frustrating as we cant find the actula reason for this issue. Can anyone please help... Thanks in Advance.. Asif

Software/Hardware used:
Windows Server 2003 SP2 , Exchange Server 2003

Answer Wiki

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Sounds like a potential DNS name resolution issue. There could also be some intermittent firewall issues blocking the SMTP connections. Check the DNS server settings first and confirm all looks healthy there. Check the relay server also for any firewall blocking connections inbound/outbound. If the issue continues, please provide more details in the discussion area of this question.

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  • Exchstart
    Hi Labnuke, Thanks for your response, as you said I have checked my DNS and cant find any issue in it and coming to my firewall I cant find any blocking for our SMTP relay . Some of the details about this issue are, 1. This issue happens randomly for some domains and when i used " Message Tracker" i found after "SMTP:started outbound TRanfer of Message" then it gives " SMTP: Non delivery msg generated" 2. We are using a Frontend server for external mails so when I check the queue after sending the mail for a domain it will be clear and there will not be any mails stuck in that queue. So i cant figure out where the exact issue and once I find that i can try to resolve it. Please help me out to resove this if u can. Thanks Asif
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  • Labnuke99
    You may need to perform a packet capture between your mail server and one of the undeliverable email domains. Can you recreate the problem on demand? If so, setup Wireshark or some other packet capture tool so it can see what transactions the sending server has with the receiving server (include any traffic to/from the sending server so you can see DNS - MX record lookups and such). This may give some indication if the DNS lookup may be failing or there may be some rejection message coming from the receiving host. Consider also checking disk and memory resources on your relay host. Look at the network utilization also to see if the capacity is used up by other services.
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