Hi @ricardo-6009 ,
Welcome to our forum.
Based on my research, I found an article which mentioned that this event is not fired if more than 16 elements are dragged for Outlook2000、Outlook20020、Outlook2003, and is not fired if more than 512 elements for Outlook 2007 with SP2, for reference only. For newer versions of Outlook, no related instructions were found for Items.ItemAdd event.
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For your another question about how to add a large number of items moving to Outlook Outbox, I have done a lot of searches but didn't find any helpful suggestions. Please kindly understand that the Outlook tag here we mainly focus on general issues about Outlook desktop client and know little about vba. In order to better solve your problem, we will remove add "office-vba-dev" tag.
Thanks for your understanding and hope your question will be resolved soon.
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