I haven't checked, but I doubt that such a specific complicated script exists.
I found this link, bit is realted to only part of the problem.
http://www.autohotkey.com/forum/topic47986.html
I did a quick test with Win Mail and I can select several emails, hit forward, and they become attachements. I sent it to myself, opened the main message and saved the attachments, they became *.EML files that can be opened with notepad. All the body of the email appears to be there, in text form. No clue about images.
You could write an AHK script to go through all the files and collect the receipt date, or send date (depending) Then you could make a list of the filenames vs the date&time and sort this list by date&time.
You could then look through this list and parse each file extracting the needed information, and writing it to a file with little --8<----8<----8<---
lines between each message.
Wow, the script would be easy for YOU to write.

The only things to check are:
Is this the way the boss wants it done?
If you go to court, will this 'hold up' in court?
What actual information is needed?
Can you d/l all the emails like this?
Would it take longer to write the code or do it manually?
Are ALL THE EMAILS available, including the ones that weren't sent to that company, but were only internal to your company?

Or you could check to see if an application exists that you can buy that already does this.

Maybe your local courthouse has someone that can give you a name of a company that designs/sells this type of software.