Had this same issue many times sending via AWS (SES). Spent dozens of hours on it with no help from Microsoft, ultimately Amazon had to sort it out with them.

Had this same issue many times sending via AWS (SES). Spent dozens of hours on it with no help from Microsoft, ultimately Amazon had to sort it out with them. Silently dropping mail (not even to the spam folder) is a maddeningly cryptic action on MS's part, and has squandered massive amounts of developer and system administrator time. If you're in a position to sway anybody from using Outlook.com, Hotmail.com, or Msn.com or Live.com I highly encourage it - MS has made a lot of lives difficult with their email policies, including NerdAdmin's here.