S3140 is Microsoft's internal reason code attached to a block, and it points at the reputation of the sending IP. You will normally see it alongside a 550 5.7.x response. Treat it the same as any Outlook reputation block: stop sending from that IP and work the delisting.
What it looks like in the log
550 5.7.1 Unfortunately, messages from [x.x.x.x] weren't sent. Please contact your Internet service provider. You can tell them that Outlook refused to accept your message. [S3140]
Why it happens
Poor IP reputation at Outlook, usually complaint-driven.
Little or no sending history from this IP to Microsoft domains.
Volume out of proportion to the IP's established reputation.
What to do
Enrol the IPs in SNDS so you can see the complaint and trap data behind the block.
Join JMRP so Outlook complaints reach your suppression pipeline.
Request delisting once, with the cause already fixed.
Warm the IP back up slowly rather than resuming at previous volume.