Hi,
Ok, you can't rely on how users send the information if you need to automate updates. You have to make a rule. If user wants his changes taken into account, he must follow a procedure else that will be a nightmare near impossible to automate. For same reason, you can't have same subject for different items.
You'll tell me that ID is in the mail body, but again, it is more complex to extract that data from body with all other data around than from subject.
So, when you receive the first mail, you create an item, no need to have information to identify that item in subject at this stage as you create it, then you send a mail back with the item ID and title as subject. From now, if user want updating item, he has to reply to that last email or send an email with the [ID:XXXX] part in subject.
Believe me, I've been advising big companies for over 25 years now, such subject comes always on the table, IT doesn't make miracle and sparing the users whatever the cost is not always the solution. Everyone has to go a little way to drive project to success. In other words, if IT does the heavy lifting, the users must also contribute their stone to the building, if only by respecting a simple rule.
Can you issue such rule to make your project a success ?
[Edit] About ID, no need to use the item ID if it looks too technical for users. But at least you need an unique identifier. You can create a column accepting unique value in which you'll set a custom ID everybody can understand, easier for them to reference it in mail subject than a number.
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