This VBA will automatically add the sender and all recipients of an email as new contacts when you replying the email in Outlook. Please do as follows:

1. Press Alt + F11 keys to open the Microsoft Visual Basic for Applications window.

2. Expand the Project1, and double click ThisOutlookSession to open it, and then paste below VBA code into the ThisOutlookSession window. See screenshot:

VBA: Auto Add Contacts from an email when replying in Outlook

Public WithEvents xExplorer As Outlook.Explorer
Public WithEvents xMailItem As Outlook.MailItem
Sub Application_Startup()
Set xExplorer = Outlook.Application.ActiveExplorer
End Sub

Private Sub xExplorer_SelectionChange()
On Error Resume Next
Set xMailItem = xExplorer.Selection.Item(1)
End Sub

Private Sub xMailItem_Reply(ByVal Response As Object, Cancel As Boolean)
Dim xNameSpace As NameSpace
Dim xSenderAddress As String
Dim xContactItems As Outlook.Items
Dim i, k As Long
Dim xFilterAddress As String
Dim xContact As Outlook.ContactItem
Dim xNewContact As Outlook.ContactItem
Dim Arr() As String
Dim ArrName() As String
Dim xArrCount As Integer
On Error Resume Next
ReDim Arr(xMailItem.Recipients.Count + 1)
ReDim ArrName(xMailItem.Recipients.Count + 1)
xSenderAddress = xMailItem.SenderEmailAddress
Arr(0) = xSenderAddress
ArrName(0) = xMailItem.SenderName
For i = LBound(Arr) + 1 To UBound(Arr) - 1
Arr(i) = xMailItem.Recipients.Item(i).Address
ArrName(i) = xMailItem.Recipients.Item(i).Name
Next i
Set xNameSpace = Outlook.Application.GetNamespace("MAPI")
Set xContactItems = xNameSpace.GetDefaultFolder(olFolderContacts).Items
For i = LBound(Arr) To UBound(Arr) - 1
For k = 1 To 3
xFilterAddress = "[Email" & k & "Address] = " & Arr(i)
Set xContact = xContactItems.Find(xFilterAddress)
If Not (xContact Is Nothing) Then
Exit For
End If
Next k
If xContact Is Nothing Then
Set xNewContact = Outlook.Application.CreateItem(olContactItem)
With xNewContact
.FullName = ArrName(i)
.Email1Address = Arr(i)
.Categories = "From Email"
.Save
End With
End If
Next i
End Sub

3. Save the VBA code, and restart your Microsoft Outlook.

From now on, when you reply an email in Outlook, this email’s sender and all recipients will be saved as new contacts automatically into the default contact folder of default email account.

How to batch add contacts from Sent Items emails/folder in Outlook?

How to bulk add contacts to contact group in Outlook?