I have two emails sent on consecutive days to our support email address.

Both came from the same person however the subject is different.  We do prefix external emails with "[EXTERNAL]" in the subject.  This is quite new and may not be relevant.  The rest of the subject is different.

CRM has matched the second email with the Case from the first email.

We have Correlation and then Smart Matching.

I found I could get the headers from Outlook and have them both side by side.  They look different but am unsure what CRM is using to match.  Again: From and To are the same, while Subject is different.

Is there anything else in this pile of stuff that I can look at to see if a match was made?

If there is no match, I presume we switch to Smart Matching?

Is there a way to test Smart Matching.  I put the regex in to an online tester and can see it stripping out "Re" and so on.  I can't see any way to test the settings to determine if the two emails would be seen as the same conversation.

Finally the three Maximum fields in the email settings are set to zero.  They have been for a long time but maybe they are wrong and a factor in the strange matching behaviour.

Any suggestions would be appreciated.

Using Server Side Sync.