I'd like a simple program to keep and view the emails with my software license keys, receipts for software purchases, etc. I also have email from other genealogy researchers that I'd like to archive. So I'd like to make copies of some of these Really Important Emails and stash them somewhere else. It isn't necessary for the archive to be a full-fledged mail agent, only that I can read the email, and copy-and-paste the information inside.
My main desktop is on Windows 8.1 at the moment; I plan to upgrade to Windows 10 before the free upgrade period ends. For Windows, I am considering a PortableApp so I could copy the archive to thumb drives along with my password stash and and other useful information. I also have Knoppix on a thumb drive, so if there's a good Linux solution, I could keep my keys on that drive, too.
At the moment, my email is in a mess. Most of the software keys will be in Thunderbird on my current computer, but I have some email in Eudora on an old Windows 98 computer, and I have multiple profiles of Thunderbird which I need to consolidate. I have a couple of Win98 programs I'd like to run in a VM, so I need to keep the keys for those, too.
What should I be considering as I evaluate the import features for these programs? My ideal program would act like Beyond Compare for files and help me sort out duplicates, but I don't know if anything like that exists for mboxes or other email formats.
One advantage of a pure viewer over a mail agent would be that I wouldn't have to link it to any of my existing email accounts before I could set it up.