//Step 1: Add reference of  "Microsoft CDO 1.21 Library" and "Outlook"

private object oMissing = System.Reflection.Missing.Value;

public string GetFromAddress ( Outlook.MailItem msg )
  {
   string strReturn = string.Empty;

   return strReturn;  
  }

You can also use VBA as following:
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'Desc: Get the from address from of the specified MailItem
'Input:
'   objMsg: The object class can be Outlook.MailItem or MAPI.Message
'Output:
'   byte array
'Remarks:
'   Since there is no function to get from address in outlook, so we use this function
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Function GetFromAddress(objMsg)
   
    ' start CDO session
    Set objSession = CreateObject("MAPI.Session")
    objSession.Logon "", "", False, False
   
    ' pass message to CDO
    strEntryID = objMsg.EntryID
    strStoreID = objMsg.Parent.StoreID
    Set objCDOMSG = objSession.GetMessage(strEntryID, strStoreID)
   
    ' get sender address
    On Error Resume Next
    strAddress = objCDOMSG.Sender.Address
    If Err = &H80070005 Then
        'handle possible security patch error
        MsgBox "The Outlook E-mail and CDO Security Patches are " & _
            "apparently installed on this machine. " & _
            "You must response Yes to the prompt about " & _
            "accessing e-mail addresses if you want to " & _
            "get the From address.", vbExclamation, _
            "GetFromAddress"
    End If
   
    GetFromAddress = strAddress
   
    Set objCDOMSG = Nothing
    objSession.Logoff
    Set objSession = Nothing
End Function