Email API
Calendar API
Contacts API 
Scheduler
Notetaker API
Google email & calendar integration
Outlook email & calendar integration
IMAP integration
Google Meet integration
Zoom integration
Microsoft Teams integration
Docs
SDKs
Tutorials
API status
Changelog
Support
Blog
Events
Guides
Inspiration gallery
Build vs. buy
Customer stories
Security & Compliance
Why Nylas
Newsroom
How to read Office 365 spam headers: A simple guide for non-admins
• 4 min read
This post is part of our “What to do when there’s no email admin” series — practical guides for anyone who suddenly finds themselves responsible for email systems they’ve never touched before.
If you missed earlier posts, start with: Understanding Microsoft 365 admin approval and Why your emails bounce (and how to fix deliverability issues).
What to do when Office365 sends your emails to spam — and how to decode what actually happened
When emails land in spam inside an Office365 mailbox, the first question users ask is: “Why?”
And unless you have a dedicated IT team, that question usually comes straight to you — the developer, support lead, or office manager who suddenly needs to act as the “email admin.”
The good news: Microsoft does tell you why an email was flagged.
The bad news: it does so in the most cryptic, acronym-heavy spam header you’ll ever see.
This guide breaks that down into something readable, so you can diagnose the issue quickly without needing deep Exchange or security expertise.
Every Office365 spam evaluation starts here.
To find it:
This header contains everything Microsoft’s anti-spam engine (EOP/MDO) used to decide whether your message was safe, suspicious, or clearly spam.
Microsoft maintains a full reference here:
https://learn.microsoft.com/en-us/defender-office-365/message-headers-eop-mdo
Look for the section that begins with IPV:.
This tells you what Microsoft thought of the sending IP address.
If you don’t see NLI or CAL, check the IP using a blacklist checker:
https://mxtoolbox.com/blacklists.aspx
Important: Nylas does not send emails or provide the SMTP server.
Emails go out through the user’s own provider, so any IP reputation issue belongs to:
If something looks suspicious, the recipient’s email admin will need to review it.
The Spam Confidence Level (SCL) explains how the message body scored during Microsoft’s analysis.
If your SCL is above 1, revise the content.
Avoid URL-shorteners, remove tracking layers, align sender domains with SPF/DKIM, and avoid overly short or overly “marketing-like” messages.
The Summary of Filter Verdicts (SFV) tells you the final decision Microsoft made.
Common values:
If you see SKS, SPM, or BLK, that explains exactly why the user found it in spam.
Here’s a quick guide:
If IPV indicates an IP problem:
Check the sending IP’s reputation and contact the domain’s mail admin or provider.
If SCL is high:
Adjust the message content. Even small text changes can improve deliverability.
If SFV indicates a block:
The sender may have been manually blocked or flagged.
If you’re still unsure:
Copy the full headers and use diagnostic tools (including AI models) to interpret the entire filtering chain.
Microsoft’s headers look intimidating, but once you know which fields matter, diagnosing spam placement becomes much more manageable.
A few quick checks — IPV, SCL, and SFV — often tell the full story.
If you or your customers run into unclear filtering results, refer back to the headers, the support article, or the Nylas Developer Docs for deeper guidance.
This post is part of our “What to do when there’s no email admin” series.
If you haven’t yet, read earlier entries:
This post is part of our ‘What to do when there’s no email admin’ series…
This post is part of our “What to do when there’s no email admin” series,…
Quick summary: Compares how developers can use the Zoom Cloud Recording API, custom Linux SDK…
Sr. Manager, Technical Support Engineering