I cannot add email from my Windows identity to Outlook. says headers too long

Question

I cannot add email from my Windows identity to Outlook. says headers too long

Florin Petrutiu 0

My stored O365 and Azure identity for the company (admin account) and my own company account, I cannot add it to Outlook. Additionally, when I try to connect to admin.microsoft.com I get an error saying that headers are too long. Outlook says it cannot find the mail servers (although it is perfectly fine from the browsers, I can see email).

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Answer 2

Salamat Shah 740 MVP

Dear Florin,

The issue is likely related to corrupted cached credentials, cookies, or a profile/authentication problem rather than the mail service itself. Try clearing browser cookies and cache for Microsoft 365/Admin Center, removing all stored Microsoft credentials from Credential Manager, and then create a new Outlook profile (Control Panel → Mail → Show Profiles → Add).

The "headers are too long" error when accessing admin.microsoft.com often indicates oversized or corrupted browser cookies. After clearing cookies, try an InPrivate/Incognito window. If Outlook still cannot discover the mail server while webmail works, run the Microsoft Support and Recovery Assistant (SaRA) and verify Autodiscover and DNS records for the tenant. If the problem persists, check whether Conditional Access, security policies, or modern authentication settings are affecting Outlook sign-in.

Florin Petrutiu 0 Reputation points

2026-07-28T08:04:34.13+00:00

I have tried Incognito, and a different browser. Same issue with headers. No creds stored in Credentials Manager. It is the main login that I log in with to my Windows machine as it is part of the organization.
Salamat Shah 740 Reputation points MVP

2026-07-28T08:08:30.69+00:00

Thanks for confirming. Since the issue persists across different browsers and InPrivate sessions, and there are no credentials stored in Credential Manager, this is unlikely to be caused by local browser cache or saved credentials.

Because this account is also used to sign in to Windows and is part of the organization, the issue may be related to the device's Entra ID/Azure AD registration, authentication tokens, or account-specific configuration. Could you confirm whether the same account can access admin.microsoft.com and be added to Outlook successfully from a different device or a non-domain-joined machine?

If the issue is limited to this device, please check Settings > Accounts > Access work or school and review the connection status. If the issue follows the account across devices, we recommend reviewing Entra ID sign-in logs and opening a Microsoft support case for further investigation.