You can view encrypted emails in Inotes by importing your notes id via Preferences>Security>Import ID . Although it seems you have to do this ahead of time or carry your notes id with you…errr. no.
How do you import the notes ID file into Inotes if you have signle sign on?
Inotes doesn’t let you do this since you cannot share the ID file.
Also once turned on users have forgotten their Notes passwords.
Therefore options seem to be :
Not to have single sign on.
Use password recovery to create a new notes id password.
Neither of which are viable options when you have lots of users now requiring this facility.
Another possibility (new in 8.5.1) is to use the ID Vault/iNotes integration to automatically import your ID into your mail file. You must know the password for your ID in the Vault in order to do this.