Direct messaging to the end user looks very similar to regular email, however behind the scenes there are a number of additional processes in place to ensure security. It is part of these security measures that identify an address as Direct enabled and thus the absence of these security measures indicate the address is a regular email address.
Please note when people say they have secure email and you are experiencing issues sending to or receiving messages from them, the reason is that secure email is not Direct. Secure email refers to having secure access to your messages, i.e. a secure web portal. However the underlying exchange of messages is just plain old regular email.
Ok, so what can I do to send / receive my Direct message?
The sender / recipient must use a Direct address, without a Direct address there is no possible way for the message exchange to occur.