[Exim] $message_id is blank

exim v4.20

I'm trying to use $message_id in the RCPT acl, however, I'm finding that it
is blank when it expands it. The manual isn't clear about this, it only
says it's not allocated to the message until the header has been
successfully received. It does say when the message is being received or
delivered, this variable is the message_id.

I would consider the SMTP session receiving a message (before DATA). I have
a nice idea, but it's blown up because of this one little detail.

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Lab tests show that use of micro$oft causes cancer in lab animals