The Deliverability Ledger · Microsoft · 10 posts
Outlook, Hotmail and Exchange Online
The 550 and 421 errors Microsoft returns, how Exchange Online Protection scores a message, SNDS and JMRP, and how to get off the Outlook blocklist.
All 10, newest first
- 421 4.7.500 “Server busy. Please try again later” — Microsoft Throttling DecodedMicrosoft's 421 4.7.500 / 4.7.501 deferrals usually mean soft throttling tied to reputation. Server load is rarely the cause. Here's how to spot the difference and recover delivery.
- 550 5.7.1 “Unfortunately, messages from [IP] weren’t sent” — How to Fix ItThe 550 5.7.1 rejection from Outlook.com almost always points at IP reputation. Here's how Microsoft decides to block, and the exact steps to get unblocked and stay that way.
- 550 5.7.708 “Access denied, traffic not accepted from this IP” — Decoded5.7.708 is the Microsoft 365 / Exchange Online cousin of S3140, but the cause is often a shared sending pool, not your domain. Here's how to diagnose and fix it.
- Exchange Online Protection (EOP) Filtering Explained: SCL, BCL, and PCL ScoresEvery message that hits a Microsoft 365 mailbox gets graded by EOP with SCL and BCL scores in the headers. Reading them turns junk-folder debugging from guesswork into evidence.
- Microsoft’s Bulk Sender Requirements: What Changed and What You Need to DoMicrosoft's bulk-sender requirements mirror Gmail and Yahoo, but with stricter enforcement timelines. If you send 5,000+ messages a day to Outlook addresses, this is the checklist.
- How to Get Delisted from the Microsoft Outlook Blacklist (Step-by-Step)Microsoft's delisting form looks simple but the approval rate plummets if you skip the prep work. Here's exactly what to fill in, what to attach, and what to do if you're denied.
- S3140 and S3150 Microsoft SMTP Errors ExplainedS3140 means Microsoft's reputation engine has listed your IP. S3150 means it listed you *and* your authentication or content looks bad. Here's how to read the codes and clear them.
- Microsoft SNDS: Reading the Data Like a ProSNDS is the most honest reputation signal Microsoft gives you, if you know what each column means. Here's how to read the dashboard column by column and what to act on first.
- Microsoft SNDS & JMRP: The Free Tools Every Sender Should Be UsingIf you send any meaningful volume to Outlook.com or Hotmail, you should be in SNDS and JMRP. They're free, take 15 minutes to set up, and tell you what Microsoft thinks of your IPs in near real time.
- Why Your Emails Land in Outlook’s Junk Folder (and How to Fix It in 2026)Outlook's junk decision is a weighted sum of reputation, authentication, content, and engagement. Here's how to influence each one, and the order to fix them in.
Other topics
- DeliverabilityThe signals inbox providers weigh, what each SMTP code and bounce type means, list hygiene, warm-up, and the numbers to measure yourself against.
- GoogleThe Gmail bounces senders hit most (421 4.7.0, 421 4.7.28, 550 5.7.26), the bulk sender requirements, Postmaster Tools, and the way back out of the spam folder.
- BlacklistsHow to tell if you are listed, why IP and domain blacklists catch different things, and what SURBL listings mean for cold email.
- AuthenticationSetting up SPF, DKIM and DMARC step by step, and choosing between a quarantine and a reject policy.
- YahooThe two refusals Yahoo and AOL send most, 421 [TSS04] for complaints and 554 5.7.9 for policy, and how to clear each.
- Cold EmailGetting cold outreach into the inbox without burning the domain it is sent from.
- ReputationThe usual reasons a sender reputation drops, and how to rebuild it.
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