Why Is Outlook (Hotmail, Outlook.com) Blocking My Emails?

Outlook blocks your email because something about the sending IP, the sending domain or the traffic pattern failed one of Microsoft's checks. The bounce tells you which one: S3150 and S3140 mean the IP is on Microsoft's internal block list, 5.7.515 means your domain failed authentication, 4.7.650 means you're being throttled for reputation, and 5.7.606 through 5.7.649 mean a Microsoft 365 banned-IP list. Each has a different fix, so start with the bounce, not with guesses.

Two Microsofts, two sets of rules

When people say "Outlook is blocking me," they usually mean one of two different systems:

• Outlook.com consumer mail: addresses at outlook.com, hotmail.com, live.com and msn.com. Filtering and reputation are run by the Outlook.com team, and its block codes look like S3150.
• Microsoft 365 (Exchange Online): business domains that host their mail with Microsoft, like jane@example.com with MX records pointing at mail.protection.outlook.com. Filtering is Exchange Online Protection, with codes like 5.7.606 and 5.7.509.

They share infrastructure, so bounces from both can come from hostnames ending in outlook.com. But they have separate block lists and separate removal paths, and mixing them up is the most common way people waste a week. Some blocks on Microsoft 365 aren't Microsoft's doing at all: the recipient's admin can block senders, and 5.7.1 Delivery not authorized often means a rule in that one organization.

Also check whether you're blocked or junked. A bounce means rejection. If the mail is accepted and lands in Junk, that's a filtering problem covered in why emails go to junk in Outlook and Hotmail.

Read the code

Full removal steps for every row are in how to get delisted by Microsoft.

Common causes

Roughly in the order I'd check them:

1. IP reputation. The most common reason for outright blocks at Outlook.com. New IPs with no history, IPs in hosting ranges with a lot of abuse, and IPs that have sent spam (yours or a previous tenant's) all end up here. If you send through an ESP's shared pool, the reputation is partly other customers'.

2. Authentication gaps. Since May 2025, Outlook.com rejects mail from high-volume domains that don't pass SPF, DKIM and aligned DMARC. The most frequent miss is DKIM: the ESP signs with its own domain, or a secondary sending tool doesn't sign at all. Run the domain through the email checker and count SPF lookups with the SPF analyzer.

3. Complaints and spam traps. Microsoft weighs junk reports heavily. Sending to old, purchased or poorly sourced lists produces complaints and trap hits, and the IP's reputation drops until blocks start. If this sounds familiar, read why your ESP can't fix your spam trap problem.

4. Volume spikes. A sudden jump in volume from an IP Microsoft knows as low-volume looks like a compromise. That's when 4.7.650 throttling and Microsoft 365's 4.7.5xx temporary restrictions show up.

5. A compromised account or server. If volume is up and you don't know why, check before you do anything else. Microsoft won't lift a block while the spam is still flowing.

How to fix it

1. Pull real bounces. Get the full SMTP response from your ESP or MTA logs, not the summary your platform shows. Note the code, the IP in brackets and the recipient domain.
2. Segment by recipient. Are only outlook.com and hotmail.com addresses bouncing, only certain business domains, or both? Consumer-only points to Outlook.com filtering. A spread across many unrelated domains suggests a public blocklist rather than Microsoft. Check which blocklists actually matter and the blocklist playbook.
3. Fix the cause before asking for removal. Authentication for 5.7.515 and 5.7.509; list cleanup, volume control or a compromise fix for reputation blocks.
4. Set up SNDS and JMRP if you control your own IPs. SNDS shows Microsoft's view of your IPs' traffic and filtering, and JMRP sends you copies of messages users mark as junk. Both are free at the Outlook.com postmaster site.
5. Request removal through the right channel. Outlook.com sender support for S-codes, the delist portal for 5.7.606 to 5.7.649, email for 5.7.511. Authentication errors have no removal process: fix the records and the next message is judged fresh.
6. Resume carefully. Start with your most engaged recipients and ramp up over days, not all at once.

How to confirm it's fixed

Send to a few Outlook.com and Hotmail addresses you control and check your logs for 250 accepted responses. Look at the headers on the delivered copies (the header analyzer helps) to confirm SPF, DKIM and DMARC all pass. Then check placement: freshly unblocked IPs often land in Junk for a while. In SNDS, watch the daily filtering results improve over the following weeks.

How to keep it from coming back

• Authenticate every service that sends as your domain, and recheck when you add a new one.
• Watch JMRP complaint reports and SNDS data weekly, not only when something breaks.
• Remove hard bounces immediately and suppress people who haven't engaged in months.
• Warm new IPs gradually and avoid sudden volume jumps to Microsoft domains.
• Keep reverse DNS (PTR) records in place on every sending IP.

When to get help

If you're bouncing at Microsoft and can't tell which system is blocking you, or you've cleaned up and requested removal and the blocks keep coming back, it's worth having an expert look at your bounces and data.

FAQ

Why does my email deliver to Gmail but not Hotmail?

Each provider runs its own reputation system, and Microsoft's is generally the most sensitive to IP reputation and complaints. Being fine at Gmail says little about how Microsoft scores the same IP.

Can I check if Microsoft has blocked my IP?

There's no public lookup. The bounce is the definitive signal, and if you control the IP, SNDS shows Microsoft's view of it.

Will moving to Microsoft 365 for my own mailboxes help me reach Outlook.com?

Not for bulk or marketing mail. Microsoft's own Exchange Online limits documentation tells customers sending bulk commercial email to use a provider that specializes in it, and the problems behind most blocks (list quality, authentication, volume patterns) travel with you to any platform.

How long do Microsoft blocks last?

Throttling (4.7.650) and Microsoft 365's 4.7.5xx restrictions are temporary. Listings like S3150 and the 5.7.606 range usually stay until the cause is fixed and you request removal.