Emails in TechDoc

I would recommend using a document category that is configured to store generations (files) encrypted for your first level of protection. This guarantees that the files are stored encrypted while at rest and cannot be accessed/unencrypted by anyone other than those who have access. This does however include admins as you've mentioned. To completely protect a file from absolutely anyone other than yourself, you would in fact need to encrypt/password protect the email before you place it into TechDoc. While passwords are easy to forget, password protecting/encrypting files before they enter TechDoc is about the best way to tackle what you've described, There are other methods to accomplish this such as using file encryption software that uses a private certificate (in lieu of a password) but even still, you still stand the risk of loosing the certificate.

If you are concerned about loosing the password/certificate, you could simply write down the password or store the certificate on a USB key and place both of these in secure storage.

Security to this level always takes a good bit of effort on the person's part and there is always an inherit risk of loosing the password, certificate, etc. After loosing this vital password, certificate, etc. it becomes impossible to access the original unencrypted file. However, this is the entire point of applying this encryption in the first place; if a encrypted file is easily recoverable without the proper credentials it buys you nothing in means of security.