I'm a support engineer in CSS. I was working with a customer who reported a mail loop error for a specific domain like contoso.com. This error was only observed in large emails.  Yeah that’s really mysterious until you figure out that the mail loop is due to size restriction on the Send connector.  I thought this was curious enough to share.

Understanding the configuration and root cause of the issue:

I initially thought that this might have been the outcome of the Edge server being configured to use an external DNS server (a DNS server that resolves external hosts). Usually, when the Edge Transport server is configured to use an external DNS, it resolves the domain name to the public IP addresses (generally pointing to itself, the external firewall, or the service provider) instead of a Hub Transport server in the Active Directory site, causing a mail loop.

Read the complete blog at http://blogs.technet.com/b/exchange/archive/2013/03/28/mysterious-mail-loop-on-edge-transport-server-check-your-size-limits.aspx

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