Can't email one external domain
Posted on 2012-09-05
We have an issue trying to send email to one external domain. This domain has a primary and secondary mail server. They do not have a load balancer. When email is received by the primary, email always transmits. When it’s received by the secondary, a connection is never made and the email is returned non-deliverable. What I'm not sure of why mail is trying to connect to the secondary and not the primary all the time. I don’t know what mechanism controls this. I’ve worked with the receiving domain admin, but the issue continues and I’m at a loss.
Exchange 2010 with DAG on VMware
Barracuda Spam Firewall where both inbound and outbound email travels through.
ASA 5510 Firewall
Here’s what I’ve done:
• Confirmed our firewall is not inspecting outbound SMTP traffic.
• Confirmed there is no issue when mail is received by primary mail server
• Confirmed the issue only happens when mail is received by secondary mail server
• Confirmed both mail servers can be resolved using internal and external DNS
• Bypassed our Barracuda Spam firewall with a direct scoped Exchange send connector to the receiving domain (Normally all outbound mail goes through our Barracuda spam firewall)
• Confirmed there is an issue when making telnet connections on port 25 to both servers from our email servers. The initial connection always fails. The second attempt is always successful.
• Confirmed with Barracuda Support there is not configuration issue with our spam server that would prevent the connection
• Confirmed that this is the only domain we have this issue with.
Any help much appreciated!