Unique descriptor

Hello all,

Is there a unique way to tell one email from another beside date/time? In other words is there a qualifier like a unique number given to each new email that is generated. When you are trying to keep track of a specific email, it is handy to be able to search this unique number/qualifier so that you can find it quickly, rather than a broad based search.

Once upon a time I used an add-on inside of Outlook that gave each email a new number that was placed in the subject and it worked but was not much liked by my clients. So I’m wondering how eM Client is built and if there is a current way to identify a specific email other than date/time.

Thank you in advance for your help.
Mike