For Each loop: Some items get skipped when looping through Outlook mailbox to delete items

Question

I wanted to develop VBA code that:

1. Loops through all email items in mailbox
2. If there are any type of other items say "Calendar Invitation" skips that item.
3. Finds out the emails with attachments
4. If attached file has ".xml" extension and a specific title in it, saves it to a directory, if not it keeps searching
5. Puts all email includes .xml attachments to "Deleted Items" folder after doing step 4 and deletes all emails in that folder by looping.

Code works perfect EXCEPT; For example

1. There are 8 email received with ".xml" file attached to each one of them in your mailbox.
2. run the code
3. you will see only 4 of the 8 items are processed successfully, other 4 remain in their positions.
4. If you run the code again, now there would be 2 items processed successfully and other 2 remain in your mailbox.

Problem: After running the code, it is supposed to process all files and deletes them all not the half of them in each run. I want it to process all items at a single run.

BTW, this code runs every time I open the Outlook.

Private Sub Application_Startup()
'Initializing Application_Startup forces the macros to be accessible from other offic apps

'Process XML emails

Dim InboxMsg As Object

Dim DeletedItems As Outlook.Folder
Dim MsgAttachment As Outlook.Attachment
Dim ns As Outlook.NameSpace
Dim Inbox As Outlook.Folder

Dim fPathTemp As String
Dim fPathXML_SEM As String
Dim fPathEmail_SEM As String
Dim i As Long
Dim xmlDoc As New MSXML2.DOMDocument60
Dim xmlTitle As MSXML2.IXMLDOMNode
Dim xmlSupNum As MSXML2.IXMLDOMNode

    'Specify the folder where the attachments will be saved
    fPathTemp = "some directory, doesn't matter"
    fPathXML_SEM = "some directory, doesn't matter"
    fPathEmail_SEM = "some directory, doesn't matter"

    'Setup Outlook
    Set ns = GetNamespace("MAPI")
    Set Inbox = ns.Folders.Item("mailbox-name").Folders("Inbox")
    Set DeletedItems = ns.Folders.Item("mailbox-name").Folders("Deleted Items")


    'Loop through all Items in Inbox, find the xml attachements and process if they are the matching reponses
    'On Error Resume Next
    For Each InboxMsg In Inbox.Items
        If InboxMsg.Class = olMail Then 'if it is a mail item

            'Check for xml attachement
            For Each MsgAttachment In InboxMsg.Attachments

                If Right(MsgAttachment.DisplayName, 3) = "xml" Then

                    'Load XML and test for the title of the file
                    MsgAttachment.SaveAsFile fPathTemp & MsgAttachment.FileName
                    xmlDoc.Load fPathTemp & MsgAttachment.FileName
                    Set xmlTitle = xmlDoc.SelectSingleNode("//title")
                    Select Case xmlTitle.Text
                        Case "specific title"
                            'Get supplier number
                            Set xmlSupNum = xmlDoc.SelectSingleNode("//supplierNum")
                            'Save the XML to the correct folder
                            MsgAttachment.SaveAsFile fPathXML_SEM & xmlSupNum.Text & "_" & Format(Date, "yyyy-mm-dd") & ".xml"
                            'Save the email to the correct folder
                            InboxMsg.SaveAs fPathEmail_SEM & xmlSupNum.Text & "_" & Format(Date, "yyyy-mm-dd") & ".msg"
                            'Delete the message
                            InboxMsg.Move DeletedItems
                        Case Else

                    End Select
                    'Delete the temp file
                    On Error Resume Next
                    Kill fPathTemp & MsgAttachment.FileName
                    On Error GoTo 0
                    'Unload xmldoc
                    Set xmlDoc = Nothing
                    Set xmlTitle = Nothing
                    Set xmlSupNum = Nothing
                End If
            Next
        End If
    Next

    'Loop through deleted items and delete
    For Each InboxMsg In DeletedItems.Items
        InboxMsg.Delete
    Next

    'Clean-up
    Set InboxMsg = Nothing
    Set DeletedItems = Nothing
    Set MsgAttachment = Nothing
    Set ns = Nothing
    Set Inbox = Nothing
    i = 0

End Sub

Answer 1

Likely cause: When you do this InboxMsg.Move, all of the messages in your inbox after the one that was moved are bumped up by one position in the list. So you end up skipping some of them. This is a major annoyance with VBA's For Each construct (and it doesn't seem to be consistent either).

Likely solution: Replace

For Each InboxMsg In Inbox.Items

with

For i = Inbox.Items.Count To 1 Step -1 'Iterates from the end backwards
    Set InboxMsg = Inbox.Items(i)

This way you iterate backward from the end of the list. When you move a message to deleted items, then it doesn't matter when the following items in the list are bumped up by one, because you've already processed them anyway.

Answer 2

It's often not a good idea to modify the contents of a (sub)set of items while looping over them. You could modify your code so that it first identifies all of the items that need to be processed, and adds them to a Collection. Then process all the items in that collection.

Basically you shouldn't be removing items from the Inbox while you're looping through its contents. First collect all the items you want to process (in your Inbox loop), then when you're done looping, process that collection of items.

Here's some pseudo-code which demonstrates this:

Private Sub Application_Startup()

    Dim collItems As New Collection

    'Start by identifying messages of interest and add them to a collection
    For Each InboxMsg In Inbox.Items
        If InboxMsg.Class = olMail Then 'if it is a mail item
            For Each MsgAttachment In InboxMsg.Attachments
                If Right(MsgAttachment.DisplayName, 3) = "xml" Then
                    collItems.Add InboxMsg
                    Exit For
                End If
            Next
        End If
    Next

    'now deal with the identified messages
    For Each InboxMsg In collItems
        ProcessMessage InboxMsg
    Next InboxMsg

    'Loop through deleted items and delete
    For Each InboxMsg In DeletedItems.Items
        InboxMsg.Delete
    Next

End Sub

Sub ProcessMessage(InboxMsg As Object)
    'deal with attachment(s) and delete message
End Sub