Alter Subject on Outbound emails through SMG

We are running Exchange Server 2013 – Enterprise edition. Currently we have an inbound content filtering rule on our Symantec Messaging Gateway appliances to where when an email arrives from the internet addressed to someone inside our domain a suffix in the message subject is added in the form of {external} for example if I send an email from an internet account to internal user with the messages Subject of “Test 123” the subject to the internal email recipient will arrive as “Test 123 {External}”. The problem we are running in to is that when an email conversation takes places between an internal user and someone on the internet the {External} suffix keeps getting added to each external reply to where after three emails the subject will look like “Test 123 {External} {External} {External}”. I have looked through SMG configuration and have even opened up a support case to ask if there is a way to get around to this issue but I was told there isn’t. I am wondering if anyone else has deployed a similar setup and if so, if anyone has been able to find a workaround?