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Over the past few hours we have seen mx1/2/3/4.hotmail.com randomly returning '554 Transaction failed' errors. Such that the MX will accept an email then seconds later another email will be rejected with the 554 error. This seems to be random across all four MXs (and the various IPs behind them).
Opened a ticket with Outlook.com deliverability and they advise they are not blocking our IPs (we monitor for spam, are registered for FBL, etc). This is happening from a number of servers in two difference DCs / IP blocks.
The 554 error is not listed on the Outlook.com postmaster pages, where various other 400/500 errors are for the 'normal' deferals / blocking.