Mail errors · Microsoft 365 and Outlook

What does “dmarc=bestguesspass” mean?

The sending domain has no DMARC record.

Last read 14 October 2026 from the sources at the foot of this page.

The fix · DNS record

Type: TXT
Name: _dmarc
Value: v=DMARC1; p=none; rua=mailto:[email protected]

The first stage. Use your own domain and a mailbox someone reads.

What does it mean?

bestguesspass is Microsoft's note that it checked as if a DMARC record existed. With no record, nothing tells mail servers to refuse mail forged in the domain's name.

What exactly does the source say?

bestguesspass: Indicates that no DMARC TXT record exists for the domain exists. If the domain had a DMARC TXT record, the DMARC check for the message would pass.
From Microsoft: Email message headers in Microsoft 365, read 14 October 2026.

Why would a small firm see it?

The domain never published a DMARC record.

How do I fix it?

Start herep=nonereports only
p=quarantinesuspicious mail to spam
Where to aimp=rejectforged mail refused

Start: reports only

v=DMARC1; p=none; rua=mailto:[email protected]

Then: suspicious mail to spam

v=DMARC1; p=quarantine; rua=mailto:[email protected]

Then: forged mail refused

v=DMARC1; p=reject; rua=mailto:[email protected]

Publish one TXT record named _dmarc with the p=none value shown here.Use a mailbox someone reads for the reports.
Read the reports for a few weeks.Each row names an address that sent mail as your domain and whether it passed.
When every service you use passes, move to p=quarantine, then to p=reject.

Use your own domain and a mailbox someone reads where the record shows yourfirm.example.

What else do the sources say?

dmarc=bestguesspass action=none
From Microsoft: Email message headers in Microsoft 365, read 14 October 2026.

none: Indicates that no DMARC TXT record exists for the sending domain in DNS.
From Microsoft: Email message headers in Microsoft 365, read 14 October 2026.

Sources

Where does this come from?

Microsoft: Email message headers in Microsoft 365learn.microsoft.comRead 14 October 2026

07 · Check my firm

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