Microsoft 5.7.606: access denied, banned sending IP
Microsoft / Outlookreputation_blockaction: pause
Microsoft 5.7.606 means your sending IP is on Outlook's block list. It is a reputation block, not a content or authentication problem, and it will not clear on its own. You need to request delisting through Microsoft and fix whatever caused the listing.
Microsoft is the slowest of the major providers to forgive and the least communicative about why. Assume recovery is measured in weeks, and that consistent low-complaint sending is the only lever that actually moves it.
What it looks like in the log
550 5.7.606 Access denied, banned sending IP [x.x.x.x]
550 5.7.606 Access denied, banned sending IP [x.x.x.x]; To request removal from this list please visit https://sender.office.com/ and follow the directions.
Why it happens
Complaints from Outlook, Hotmail or Live recipients.
Hitting spam traps in a Microsoft-operated domain.
A sharp volume increase from an IP with no history at Microsoft.
Inheriting a range that was abused before you had it.
What to do
Submit a delisting request through the Microsoft sender support form. Do this once, not repeatedly.
Register the IPs for SNDS and JMRP first, so you can see complaint data and receive feedback.
Fix the cause before requesting removal. Microsoft relists quickly, and repeat listings are harder to clear.
Route Microsoft traffic off the affected range while you work the delisting.