[Exim] Adding "Seen" Headers

Hello Everyone;

I would like to uniquely tag a message after it's been "seen" by a
transport. I was thinking of something like this:

headers_add = X-Done-Spamcheck: ${md5:thisisthespamsecret$h_Message-ID:}

Where "thisisthespamsecret" would be the "secret" and the Message-ID
would be the unique bit of information in an email. I'm worried that
Message-ID may be null at some point though? Is there a better string
expansion to use as some sort of unique identifier?

Thanks,

-Craig