Free tool
Microsoft 365 tenant lookup
Paste a domain, an email address, a tenant ID or an Azure subscription ID to see the tenant behind it, or type a name to check whether name.onmicrosoft.com is still free.
No sign-in, and lookups are not saved. If you paste an email address, only the domain leaves your browser.
Where each answer comes from
Every answer comes from Microsoft or from public DNS. The tool never signs in to the tenant you look up, so it cannot see anything inside it.
Domain or email to tenant ID
Microsoft publishes a sign-in configuration for every domain verified in Microsoft Entra ID, and the tenant ID is part of its issuer address. The realm answer adds whether sign-in is managed by Entra or federated, and the organization name shown at sign-in.
login.microsoftonline.com/{domain}/v2.0/.well-known/openid-configurationSubscription ID to tenant
Ask Azure Resource Manager about a subscription without signing in and it refuses, naming the tenant you would have to sign in to. That tenant is the one that owns the subscription.
management.azure.com/subscriptions/{id}, the WWW-Authenticate headerIs a tenant name free?
Every tenant gets an initial domain, name.onmicrosoft.com, when it is created. If Microsoft publishes a sign-in configuration for that domain, the name is taken.
login.microsoftonline.com/{name}.onmicrosoft.com/v2.0/.well-known/openid-configurationMicrosoft 365 in public DNS
Exchange Online, Autodiscover, SPF, DKIM, Teams, Entra device registration and Intune each publish DNS records with fixed Microsoft targets. The DKIM record also carries the tenant’s initial onmicrosoft.com domain.
MX, TXT, CNAME and SRV records for the domainOrganization name and default domain
Once the tenant ID is known, Sentsai asks Microsoft Graph for the name and default domain Microsoft holds for that tenant. To ask, Sentsai signs in to its own tenant with an app that can read only that basic information. It never signs in to the tenant you look up.
graph.microsoft.com/v1.0/tenantRelationships/findTenantInformationByTenantIdCommon questions
What is a Microsoft 365 tenant ID?
A GUID that identifies your organization’s Microsoft Entra ID directory. Microsoft 365, Azure and Dynamics 365 all sit on that directory, so Microsoft support, partners and app vendors ask for the tenant ID to find your organization.
Where else can I find my own tenant ID?
In the Microsoft Entra admin center, on the Overview page under Identity, and in the Azure portal on the Microsoft Entra ID overview. Both show it beside the primary domain.
Can a name show as available and still be refused?
Yes. This check sees only names an existing tenant already uses. Microsoft makes the final check when the tenant is created and can refuse a name for its own reasons, so keep a second choice ready.
Why is a Microsoft 365 record missing for a domain I know uses it?
A missing record is not proof of absence. Mail can pass through a filtering gateway before it reaches Exchange Online, DKIM can use other selectors, and many Teams-only organizations never publish the older Skype for Business records. A found record is strong evidence; a missing one is not evidence either way.
Does Sentsai keep what I look up?
No. Lookups are not written to a database or a log, and when you paste an email address only the domain is sent. The answer is held in memory for ten minutes so a repeat lookup is quick, then it is gone.
Found your tenant? See what it pays for and nobody uses.
A free, read-only Sentsai scan checks your Microsoft 365 tenant for unassigned, idle and duplicated licenses, with results in minutes.
