Some bounce notifications don't have the DSN attached.

When a bounce DSN is received, it is saved in the message store so it can be attached to any admin notice that might be sent as a result of the DSN. The issue is if the DSN is a bounce for multiple users, the process for the first user deletes the DSN from the message store so it isn't available for the notices for the subsequent users.

Processing should not delete the DSN, but just allow it to be eventually deleted upon expiration.