What system can I create so that all emails are required to go through web-form, but a "reply" function still works?

Alright this is a basic question, but I'm uncertain of the infrastructure or technical execution.

Say you get 1 million emails per year from customers.

For stats, reporting, efficiency, rerouting, whatever --- you've elected to pass all communication through a basic web form that lists the general department/ issue, etc... a basic few facts that makes Customer Service life easier.

Here's the problem. Once you respond to the inquiry, using a real company email address --- a lot of people just start using that instead of the form in the future. So much so that almost 60-70% of contacts ORIGINATE from your unpublished email address (unpublished, except for when you reply, so they can also hit reply).

Obviously a no-reply doesn't quite work, but some conversations require a chain of emails.

I did see an interesting model from an insurance company ... they say please do not reply ... click this link to expound on your inquiry ... and it takes you to a link with a URL that looks like a random encrypted string.

Example: asp?incidentID=%7Cb1f4792f-2dbb-56a3-b46a-0815f5938881%7D

I'm not certain how to implement this personally, though. Might be kind of annoying to force replies this way, but was wondering how to do it.