If you have activated our automatic encryption for the completely encrypted mailbox, you can no longer determine whether e-mails were encrypted by us or by the sender. But it is possible you might like to find out.
You can do this with a simple trick: Generate two GPG keys. The first is your real key. You distribute this key to your contacts and have it signed if necessary. Use the second key for our automatically encrypted mailbox only. Then, based on the key ID, you can determine who encrypted the e-mail – the sender or mailbox.org.