Member Avatar for pritaeas

How would I know if an e-mail originates from Office 365?

I know I can look at the oldest Received header of a message to see where it originates from, but how would I use that header value to know it was sent from an Office 365 account?

It looks like the TLD exchangelabs.com is a candidate, but I'd like to know for sure.

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Short answer: a single relay hostname alone is not definitive. was right to inspect the oldest relay, noted a first-hop look can point to a consumer service, and is correct that clients can be configured to use any SMTP host. The reliable approach is to combine (1) the earliest relay IP ownership, (2) standard mail-auth results, and (3) Exchange/Microsoft-specific transport headers — together they give a high-confidence answer.

A practical checklist:

  • Find the earliest external hop and note the sending IP (the first relay outside the sender�s environment).
  • Do a reverse-DNS and WHOIS/ASN lookup on that IP to see if the netblock belongs to Microsoft.
  • Inspect authentication headers (Authentication-Results): look for SPF pass, a valid DKIM signature that aligns with the From header, and DMARC alignment. Passing auth results from Microsoft-owned infrastructure is strong evidence.
  • Look for Exchange Online transport headers (the X-MS-* family added inside Microsoft mailflow) — their presence supports Office 365 handling.
  • Cross-check the sending IP against Microsoft�s published mail IP ranges (see Microsoft�s Office 365 IP web service) to confirm ownership.

Cautions: hybrid setups, partner relays, or third-party mail services authorized by a tenant can make attribution ambiguous. For incident-level certainty, correlate with tenant audit/mail trace logs from the sender�s Microsoft 365 admin or request provider logs. For published ranges and guidance on email authentication see Microsoft documentation: Office 365 IP Address and URL web service and .

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Member Avatar for wwwalker

I think it is outlook.com as first hop from server to your mailbox.

Member Avatar for pritaeas

I think it is outlook.com

That's what I thought before looking into any e-mails. Did not find a single instance.

Member Avatar for JamesCherrill

In Outlook you can configure the outgoing server(s) to be whatever you want (eg Google). There's absolutely no requirement to use outlook.com or any other particular server.

Member Avatar for pritaeas

That I understand, I am not trying to determine whether it was sent with the Outlook application, but with Outlook/Office SMTP (server).

If it was sent with GMail then the first/oldest Received header would contain the TLD google.com or gmail.com (I checked).

Edited by pritaeas
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