This will allow you to set any number of days you like.
Sub Set_FollowUp()
Dim numDays As Double
Dim uPrompt As String
Dim MyMailItem As Object
On Error Resume Next
If ActiveInspector.currentItem.Class = olMail Then
Set MyMailItem = ActiveInspector.currentItem
End If
If MyMailItem Is Nothing Then
' Might be in the explorer window
If (ActiveExplorer.selection.Count = 1) And _
(ActiveExplorer.selection.Item(1).Class = olMail) Then
Set MyMailItem = ActiveExplorer.selection.Item(1)
End If
End If
If MyMailItem Is Nothing Then
MsgBox "Problem." & vbCr & vbCr & "Try again " & _
"under one of the following conditions:" & vbCr & _
"-- You are viewing a single message." & vbCr & _
"-- You have only one message selected.", _
vbInformation
Exit Sub
End If
MyMailItem.FlagDueBy = Now + 3
' *** optional code ***
'uPrompt = "Follow-Up how many days from now? Decimals allowed."
'numDays = InputBox(prompt:=uPrompt, Default:=3)
'MyMailItem.FlagDueBy = Now + numDays
'MyMailItem.FlagRequest = "Customized Follow up"
' *** end of optional code ***
MyMailItem.Save
End Sub