You can drag an email from Microsoft Outlook or indeed from Thunderbird into the Net-Cabinet folder. You will then be prompted to add Tags. The email is then saved to your document repository and is available for search and review.
If you open the email it will open in the appropriate application so that you can read, reply or forward as you need.
Attachments are handled as part of the email message, but you can equally drag and drop the attachment from the email to the Net-Cabinet folder to store separately.