Appears the NewMail event is not appropriate for this purpose. ItemAdd is recommended.

https://msdn.microsoft.com/EN-US/library/ff869202.aspx

"The NewMail event fires when new messages arrive in the Inbox and before client rule processing occurs. If you want to process items that arrive in the Inbox, consider using the ItemAdd event on the collection of items in the Inbox. The ItemAdd event passes a reference to each item that is added to a folder."

You could try either ItemAdd or NewMailEx described here. Avoid the buggy rules with "run a script" option. http://www.outlookcode.com/article.aspx?id=62