Are you looking for a setting to do this or are you willing to work with a macro? If you're open to working with macros, see below and reply back with questions.
Public WithEvents goInspectors As Outlook.Inspectors
Public WithEvents myMailItem As Outlook.MailItem
Private Sub Application_Startup()
Initialize_Inspector
End Sub
Private Sub Initialize_Inspector()
Set goInspectors = Outlook.Application.Inspectors
End Sub
Private Sub goInspectors_NewInspector(ByVal Inspector As Inspector)
If Inspector.currentItem.Class = olMail Then
Set myMailItem = Inspector.currentItem
End If
End Sub
Private Sub myMailItem_PropertyChange(ByVal Name As String)
'The variable below should be modified for your situation.
'If you are in an Exchange environment, then you can use "last name, firstname"(caps-sensitive).
'If the the recipient is not in Outlook's address list, use "person@email.com"
customSignatureFor = "Lastname, Firstname"
'Use vbCrLf to account for enter/returns
oldSignature = "Respectfully," & vbCrLf & vbCrLf & "Phillip"
newSignature = "v/r," & vbcrlf & "Phil"
If Name = "To" Then
For i = 1 To myMailItem.Recipients.count
If InStr(myMailItem.Recipients(i), customSignatureFor) > 0 Then
tempstring = Replace(myMailItem.Body, oldSignature, newSignature)
myMailItem.Body = tempstring
End If
Next
End If
End Sub