title: Can events written via EWS API not make it to the mailbox, despite being accepted successfully by the API?
body: <p>I have an application which writes lots (millions) of calendar entries to mailboxes for large organisations. Occasionally, Office 365 EWS API will accept a batch of entries, return success (not error) codes, and the entries fail to make into mailboxes.</p>
  <p>Microsoft support don't (appear to) have a public-facing API support team so the usual Microsoft support routes just say either &quot;third-party application&quot; or &quot;we don't have a support team you can speak to&quot; ... so I'm a bit stuck. This does not appear to be a failure of the app, as I can see from the trace that it writes successfully and is given a change key back. And this only happens over a short period of time (say, all writes in a 30 min window have this problem).</p>
  <p>I'm a bit stuck as to where to go here, as there's no error, just occasional and undesirable behaviour. It could even not be the API that's at fault, and could be just a sync error between EWS and mailbox stores. But, as it's Office 365, I can't see this.</p>
  <p>Application is .Net 3.5 if it helps; very stable installs, runs fine for years, just occasionally has this problem... with just one customer...</p>