This question looks like it should have been answered here: Outlook VBA Save Copy Sent Item
I am unable to find the referenced "Items.ItemAdd" event amongst the list of events in the VB Editor for Outlook 2016.
Application.ItemSend is located in "ThisOutLookSession". The script I came up with looks 90% similar to the one posted in the original question and I have the same problem. The mail item is saved in it's pre-send condition.
I can't use "MailItem.SaveSentMessageFolder" as I am trying to move the sent mail to a PST.
The Event is explained here: https://msdn.microsoft.com/en-us/vba/outlook-vba/articles/items-itemadd-event-outlook
In case someone asks "Why not use the AutoArchive Function?", my organization has some terrible group policies set up for both Sent Items and AutoArchive. I can't wade through the mess to get AutoArchive for sent items set up.