A user receives the error "Cannot find external name: SIGNENCRYPTPOLICY" when opening their mail file. Hit OK to the error, the file will open and the user can continue working without issue.
The design of the database was missing the Database Script SIGNENCRYPTPOLICY.
Resolving the problem
Replace the design of the mail file with the stock template. In Notes, go to File > Application > Replace design... using the standard Mail template.
In this particular case the template had a custom database script added to the Database Script section of the template. The Database Script section was protected from being refreshed or replaced so a replace design did not work initially. Once "Prohibit design refresh or replace to modify" was removed, the design replaced again, the SIGNENCRYPTPOLICY script got added back and the mail file opened without error.