You need to define strsubject e.g.
Code:
Dim strSubject As String 'this line already in macro
Dim oPara As Range
Dim lngPara As Long
For lngPara = 1 To ActiveDocument.Paragraphs.Count
Set oPara = ActiveDocument.Paragraphs(lngPara).Range
If Left(LCase(oPara.Text), 3) = "re:" Then
oPara.End = oPara.End - 1
oPara.MoveStartUntil Chr(32)
oPara.Start = oPara.Start + 1
strSubject = oPara.Text
Exit For
End If
Next lngPara