It is relatively easy to read the body of an e-mail and extract data from it. This is covered at http://www.gmayor.com/extract_data_from_email.htm, however in this instance you don't want to send the information to Excel, but use it to create a reply to the message, as shown below. You would run the main script from a rule to identify the messages, but I have included a test macro that you can run manually with a message selected.
Option Explicit
Sub SendAutoReply(olItem As MailItem)
Dim vText As Variant
Dim sText As String
Dim sAddr As String
Dim vItem As Variant
Dim i As Long
Dim olOutMail As MailItem
Const strTemplate As String = "C:\Path\autoreply.oft" 'Your autoreply template
'Process the message
With olItem
sText = olItem.Body
vText = Split(sText, Chr(13))
'Check each line of text in the message body
For i = UBound(vText) To 0 Step -1
If InStr(1, vText(i), "You used the following email address:") > 0 Then
vItem = Split(vText(i), Chr(58))
sAddr = Trim(vItem(1))
Set olOutMail = CreateItemFromTemplate(strTemplate)
With olOutMail
.To = sAddr
.Display '(change to .Send after testing)
End With
Exit For
End If
Next i
End With
lbl_Exit:
Set olOutMail = Nothing
Exit Sub
End Sub
Sub TestMacro()
Dim olMsg As MailItem
On Error Resume Next
Set olMsg = ActiveExplorer.Selection.Item(1)
SendAutoReply olMsg
lbl_Exit:
Exit Sub
End Sub