Read regimail in Outlook 

To read regimails in Outlook Classic, you must convert the message into an ordinary email. The easiest way to do this is to select the message, open the regimail menu and click on "Open/read regimail".
 

Once the decryption has been successfully completed, you will get displayed a decrypted copy with the addition (regify converted) in the subject line. 

If you convert the same message again later and there is already a decrypted copy in the same folder, it will not be decrypted again, but the already decrypted copy will be displayed directly. 

If you are already viewing the email in a separate window, you can also find "Open/read regimail" in the "regify" menu in the ribbon: