What I have seen work best, is a communication to the user explaining that we are in the process of migrating.An audit review of the user’s mail has uncovered that there are encrypted emails within their mail and without encryption they cannot be migrated.In both of these cases when a message is sent to a user, only the intended recipient can open it.This is a security feature of Lotus Notes and Domino that prevents an Administrator using an ID with Elevated Privileges or a Server ID from accessing messages with sensitive content.
The user can then either exit the email or a button based in Lotus Notes can then use the end user’s ID File to remove the encryption from the messages. This is part of the basic functionality of Binary Tree’s CMT For Exchange.
The other option is to have the end user decrypt this email.
My experience is that the end user has their own job to do, truly does not care about the migration and the most certainly they do not want to do the job for you.
An altrernative method is to use a Notes Database as a vessel to maintain the Notes Encryption.
These messages can be added to a small Lotus Notes database with the encryption in place.