Problem Statement : Mails to a particular domain 'B' not delivered if size is above 1 Mb. However the target domain 'B' receives mails from other domain without any problems
Our Network : Comprises of a sinlge 1700 Cisco Router. The network traffic includes SSL VPN tunnel also besides the regular FTP, http, SMTP.
Explanation : We noticed that mails to a particular domain 'B' was not getting delivered. On close scrutiny, we found that the window size was reducing as the mail was getting delivered. In all probability, this was attributed to a rougue router which was in the mail path.
MTU for our router happens to be about 1472, while the same is 1412 for the target domain 'B'.
Further, there were no rules in the content filtering tools at either end that was blocking the mail
One of the research led to a link http://www.postfix.org/faq.html. (Mail fails consistently with timeout or lost connection) This points to "ICMP MUST FRAGMENT" to be set on the router.
My quesiton is how do we trace this rouge router to enable the setting ?
Response much appreciated.
Thanks in advance.