Try Eudora OSE, I’ve been using it when I lost the ability to use Outlook Express.
It is able to maintain the saved email folder structure very well. I happen to keep almost every personal email that I have saved and I have organized them into separate folders based on senders name and type of mail such as purchases by sender or personal emails, or by various of information types.
I forgot to mention that I use a POP3 email server so I can keep all of my emails on my own personal machine and not out on the ‘cloud’ where they can be ‘accidentally’ released to the public. I appreciate my own security and since I rarely leave the house it is always available to me to look up previous discussions.
If necessary while I am away, I can use something like ‘MyPC’ to access my home computer. I also somewhat regularly backup my emails to my laptop so I can also retrieve past emails that way.