Using AppleScript to open Mail by MessageID
Mail is not smart enough to just tell it to open a document by just its message id in the manner you're trying to do it. You need to tell Mail where to look for it, as in what mailbox it's in.

The following example worked for me when MessageID was set to one in my Inbox using its proper message id however in this example I'm using the message id shown in your OP.

set MessageID to "DB43DAF1-7B87-4DE5-99DB-EC11CB0F6FF7@gmail.com"

tell application "Mail"
    activate
    open (first message of inbox whose message id = MessageID)
end tell


Note that the example code above is limited in that it doesn't gracefully trap an error, so the example code below adds a try statement  with an error statement to handle an error gracefully.

set MessageID to "DB43DAF1-7B87-4DE5-99DB-EC11CB0F6FF7@gmail.com"

tell application "Mail"
    activate
    try
        open (first message of inbox whose message id = MessageID)
    on error eStr number eNum
        display dialog eStr &amp; " Number: " &amp; eNum buttons {"OK"} default button 1
        return
    end try
end tell