Send email to newly assigned resource

I send, with a macro in MS Project, an Outlook email when a resource is assigned.

I want to notify only the resources that have been assigned since the last time the macro was run.

This code emails every assignee the task they are assigned every time it is run.

    Dim oProject As Project
    Dim oTask As Task
    Dim oResource As Resource
    Dim oMail As Object
    
    
    ' Get current project
    Set oProject = ActiveProject

    ' Loop through each task in the project
    For Each oTask In oProject.Tasks

        ' Check for new assignments on this task
       If oTask.Resources.Count > 0 Then
      
            ' Loop through each resource assigned to the task
            For Each oResource In oTask.Resources
                ' Get the email address of the resource
                ' (You'll need to store email addresses in the project)
                ' Use the resource's email address to create the email
                Set oMail = CreateObject("Outlook.Application").CreateItem(0)
                With oMail
                    .To = oResource.EMailAddress
                    .Subject = "New Task Assignment"
                    .Body = "You have been assigned to task: " & oTask.Name
                    .Send  ' Or .Display
                End With
            Next oResource
        End If
    
    Next oTask

End Sub

Rather than loop through tasks, you can loop through resources, and then loop through each resource's assignments. Working at the assignment level, use a flag field to indicate whether or not this assignment has already been emailed.

For example:

Sub EmailNewAssignments()

    Dim oOutlook As Object
    Set oOutlook = CreateObject("Outlook.Application")
                    
    Dim res As Resource
    For Each res In ActiveProject.Resources
    
        Dim taskList As String
        taskList = vbNullString
        
        Dim asn As Assignment
        For Each asn In res.Assignments
            If Not asn.Flag1 Then
                taskList = taskList & vbCrLf & "Task: " & asn.Task.Name
                asn.Flag1 = True
            End If
        Next asn
        
        If Len(taskList) > 0 Then
            Dim oMail As Object
            Set oMail = oOutlook.CreateItem(0)
            With oMail
                .To = res.EMailAddress
                .Subject = "New Task Assignments"
                .Body = "You have been assigned to the following tasks: " & taskList
                .Send
            End With
        End If
    Next res

End Sub

Sub ClearAllAssignmentFlags()

    Dim res As Resource
    For Each res In ActiveProject.Resources
    
        Dim asn As Assignment
        For Each asn In res.Assignments
            asn.Flag1 = False
        Next asn
        
    Next res

End Sub