[OUTLOOK] Problem using UserProperties.Add method

Hello all. I'm writing a VBA script to extract the email subject from certain mails. I want to extract the title and then add a UserProperty "Read" to the mails to ensure that the script does not consider them again.

However, this does not seem to be working because every time I restart the macro, the already read mails get read again and again. What could be going wrong?

Also, how do I set a default value for the UserProperty?
Here is my code :

Code:

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Sub myOlApp_NewMail()
    Dim WordArray() As String
    k = 0
    l = 0
    Dim myOlApp As New Outlook.Application  'Application Outlook
    Set MonNSpace = myOlApp.GetNamespace("MAPI")
Set FldDossier = MonNSpace.PickFolder
Debug.Print "New run"
For i = FldDossier.Items.Count To 1 Step -1
        On Error Resume Next
    If Mail.Subject = "Read Me" Then
    Set CheckProperty = MonMail.UserProperties.Find("Read")
    If CheckProperty.Value = "No" Then
        Property = Mail.UserProperties.Add("Read", olText)
        Property.Value = "Yes"
        Mail.Save
        Debug.Print "Mail not read"
  Else
        Debug.Print "Mail read"
    Next i
End Sub

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