iNotes user cannot read internet signed mails send to Mail-in DB mail file

UserA opens the signed mail-in iNotes - so far so good, the Notes ID for UserA is attached to the profile $shimmerid.
Now UserA is out of the office and UserB reads the signed external mail within iNotes - but iNotes is asking for the Notes ID password from UserA attached to the profile $shimmerid. As UserB doesn't know the password from UserA and enters his password, he is endless prompted to provide the password and cannot open the signed mail. Because the mail is signed but not encrypted, iNotes should still be able to display the mail content (possible to show a message that the Notes ID cannot be accessed - but this is not really needed).