Issue-key gets lost on reply

[Martin Molt]

When we send an E-Mail via App to a third party (not the customer itself) and that thrid party replies (with their own issue key in headline), its matched as a comment, because our issue-key is in the headline too. But the third party has its own issue key, which gets lost as the headline isn't mapped in comments. When we reply the third party again, we will open another request at their side. 

Is there any way to get around this?