I use MailStore to archive my emails. The program takes the mails from the outlook database and saves it in another format in a seperate directory (can't give you any technical details how this all works, sorry). MailStore offers a feature to open archived emails again in Outlook to view them. If the email is encrypted, I can't view it anymore because GpgOL does not detect that it is encrypted and instead says it's insecure. This results in emails being unreadable.