What do S3140 and S3150 mean in a Microsoft bounce?

They are Microsoft's internal reason codes for an IP on its own block list. S3140 usually accompanies "part of their network is on our block list" and S3150 "blocked using ... Spamhaus" or a reputation block; both are resolved through the sender support portal rather than Spamhaus.

How long does Microsoft take to delist an IP?

Submissions through sender.office.com are typically answered in 24 to 48 hours. If the first reply says the IP does not qualify, it is because Microsoft still sees spam-like traffic; fix that and resubmit.

Will mail to Gmail bounce too?

If the listing is at Spamhaus, probably yes, since most receivers consult it. Microsoft's own list only affects Outlook.com, Hotmail, Live and Microsoft 365 recipients.

550 5.7.1 Service unavailable, Client host blocked using Spamhaus (S3140, S3150)

Microsoft refused the connection because the sending IP is on a blocklist. Outlook.com replies with an S31xx code in brackets: S3140 and S3150 mean the IP is on Microsoft's own list, S3130 that it is on Spamhaus. The wording "Unfortunately, messages from [ip] weren't sent. Please contact your Internet service provider since part of their network is on our block list" is the consumer Outlook.com version of the same thing.

What the server replies

550 5.7.1 Service unavailable; Client host [203.0.113.10] blocked using Spamhaus. To request removal from this list see https://www.spamhaus.org/query/ip/203.0.113.10 (AS3130). [AM0PR06CA0012.eurprd06.prod.outlook.com]

Sent by: Outlook.com and Microsoft 365
Type: PermanentRetry fails
About: Reputation and blocklists
Enhanced code: 5.7.1

Who sends it

Exchange Online Protection (for Microsoft 365 tenants) and Outlook.com inbound servers, at connection or MAIL FROM, before any content is read. The AM0PR, DB8PR, SN1PR style hostname at the end identifies the receiving server.

Why it happens

The IP is listed on Spamhaus ZEN (SBL, XBL or PBL) because of spam from it, a malware infection, or because it sits in a residential range that should not send mail directly.
The IP is on Microsoft's internal blocklist after Outlook.com users reported mail from it.
A neighbouring IP in the same /24 was sending spam and Microsoft blocked the range.
The IP was recently assigned to you and carries its previous owner's reputation.

How to fix it

Look the IP up
Check the IP at check.spamhaus.org. A PBL listing means the range is marked as not meant to send mail; relay through your ISP or a provider instead. An SBL or XBL listing names a reason you have to fix before removal.
Stop the cause before delisting
If a host on your network is infected or an account is compromised, delisting without fixing it gets you re-listed within days. Rotate the credentials, patch the host, confirm the outbound queue is clean.
Request removal
Spamhaus listings are removed through their lookup page once the problem is gone. For S3140 and S3150, use Microsoft's sender support form; replies normally arrive within one to two business days.
Ask for a PTR and authenticate
Microsoft weighs reverse DNS, SPF and DKIM when deciding how much to trust a newly delisted IP. Have all three in place before the retry.

Check it yourself

These free tools show the records and connections behind this code for your own domain or server.

Related codes

Sources

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