Why emails get rejected by Microsoft

🚨 Sending emails? You need to read this! Here is what you don't know: Every third party DKIM checker still says pass. Microsoft bounces. Here’s the data behind the break and a three phase playbook you can ship today. What the data says: • False DKIM fails, even when SPF and DMARC pass elsewhere Traces: Gmail / Yahoo = pass, Microsoft = fail • Inconsistent behaviour: identical messages, one lands, one bounces reports show“some messages bounce; others don’t” under identical conditions • Stricter alignment: Microsoft may require the same root domain in From: and DKIM our guidance: “check alignment” • Content sensitivity: non-ASCII characters or heavy MIME encoding trigger rejection “Content sensitivity” is a huge factor • Key age: selectors last rotated before March 2025 fail more often “Key age” is a note • Header gaps: signing a header that later disappears (for example Message-ID) breaks DKIM pay attention to “Header inconsistencies”. • Volume spotlight: about 1.8 % of Microsoft bounces in May June trace to auth issues • Forwarding risk: Outlook recommends ARC to preserve auth during forwarding Microsoft documentation confirms that starting May 2025, any message tripping these stricter checks is rejected outright with 5.7.515. Phase 1 – Triage (today, < 30 min) - Verify the trio SPF ≤ 10 look-ups · single-line 2048-bit DKIM · DMARC aligned with SPF or DKIM. - Reproduce the failure Send the same message to one Gmail and one Outlook seed. Gmail lands, Outlook bounces → Microsoft-side break, not DNS drift. - Package the evidence Keep the Outlook NDR, Gmail headers, and DNS screenshots. Microsoft support asks for them first. Phase 2 – Stabilise (next 24 h) - Force strict alignment: sign with the exact root domain shown in From: - Rotate stale keys: replace selectors older than 6–12 months; retire only after DNS propagates. - Sign only stable headers: keep from, to, subject, date, mime-version; drop volatile lines like message-id, received, or X-. - ASCII-clean a control send: strip smart quotes, accents, emojis, and any footer injected after signing. - Add ARC if you forward mail: preserves the auth chain Microsoft expects. Phase 3 – Harden (next 7 days) Segment Microsoft traffic: place Outlook/Hotmail addresses on their own IP pool or sub-domain. Wire in live telemetry: · DMARC aggregates filtered for Outlook-only DKIM = fail rows. · Microsoft SNDS and JMRP for IP- and complaint-level signals; send real-time alerts to Slack or email. Quarterly auth audits: every new SaaS sender must pass a staging mailbox test before going live. Escalate with a full packet: NDR + passing Gmail header + DNS records + timeline. Microsoft’s “Fix 550 5.7.515” article lists these as required. Spot new patterns? Comment below more data means a stronger playbook for everyone.

If you’re still sending email from an onmicrosoft.com address, Microsoft is tightening the rules. This matters because your messages could start getting throttled or blocked, which means invoices, password resets, and customer updates might never arrive. Microsoft’s goal is to stop spammers who spin up fresh tenants and abuse the shared onmicrosoft.com domain. But the side effect is real organizations will see lower deliverability and limits on bulk or automated sends until they move to a proper, verified domain. What’s changing? Microsoft is putting sending limits and stricter checks on any email that leaves an onmicrosoft.com address. Because it’s a shared domain used by millions, one bad actor can hurt the reputation for everyone. The fix is simple but urgent: switch to your own branded domain and set up modern email authentication (SPF, DKIM, and DMARC). That tells receiving mail systems, “Yes, this is really us,” and helps keep your mail out of spam and off block lists. What should you do now? Audit where onmicrosoft.com shows up—service accounts, no-reply inboxes, ticketing tools, scanners, CRM alerts, and scripts. Register or connect your custom domain, add the DNS records, and rotate apps and automations over to the new addresses. Test mail flow, watch for bounce backs, and update address books, forms, and templates. Train your team so they know which sender addresses are approved going forward. A little cleanup today will save a lot of missed messages tomorrow. #Microsoft365 #EmailSecurity #ITAdmin #ChangeYourPassword Follow me for regular updates on Microsoft 365 changes, security tips, and clean-up checklists that keep your org’s email flowing.

New Outlook rules take effect today & Microsoft will begin rejecting / bouncing emails from domains that lack proper authentication. If your domain isn’t set up with SPF, DKIM, and DMARC, your emails may not reach any Outlook, Hotmail, or Live users. This is especially critical for anyone sending emails to more than 5,000 recipients at once. Initially, Microsoft announced that unauthenticated emails would be filtered into spam. However, last week they changed course and decided to enforce rejections right from the start. As a result, many companies will likely see more rejected / bounced emails, flagged with the 550 5.7.515 error code. Whatever system you use for mail distribution, make sure your domain is properly authenticated with their infrastructure. Even if you think it is, it's worth double-checking the authentication settings and analyzing your #DMARC reports from the past month to ensure all existing mail streams are properly configured. #Microsoft #EmailDeliverability