SecurePIM (v6.9.1-armabi-v7a) sometimes does not show our messages because of connection errors.
These happen to me when I switch between test mails very regularly.
In that case SecurePIM also no longer syncs mails. So the best way to test for a broken state is to send a mail to yourself because SecurePIM does not often show connection errors when syncing. They are shown only sometimes. Even restarting the App does not unblock it. Only waiting seems to help.
This seems to happen more often with mails that we classify as IPM.Note.SMIME so as a workaround we also classify encrypted Mails now as IPM.Note.SMIME.MultipartSigned. Maybe that helps.