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The Outlook Junk E-mail filter is sensitive to a large number of identical emails arriving at one time and once the threshold is reached will flag the rest of the emails as Junk. This may be a factor, but if your Outlook Server is managed by a third party, it may be difficult to customize the settings that affect this. However, if you white list the sender, it should by-pass this filter.
You may also want to consider throttling your email as described in the Throttle Email Delivery help article.