Error 403 - Forbidden The web app you have attempted to reach has blocked your access.

The 403 Forbidden message with “The web app you have attempted to reach has blocked your access” indicates that access to the site is being blocked at the web app level, not that the account is locked or the password is wrong.

In the provided context, this error appears when a backend web app is intentionally configured to block public internet access. From outside the allowed network, DNS resolves to the public IP and any direct access attempt returns a 403 Forbidden page stating that the web app has blocked access. This behavior is by design when public access is disabled.

Applied to this scenario, the most likely causes based on the context are:

1. Access to the Office 365 web app endpoint is being blocked for the current network path (for example, by network configuration, DNS, or a security control that makes the service behave like a locked-down backend app).
2. The error is consistent across devices (phones and laptops), which suggests a shared network or DNS path rather than a device-specific problem.

Actionable checks based on the context pattern:

1. Try a different network
   • Connect one device to a different network (for example, mobile data instead of home Wi‑Fi) and browse to the Office 365 portal.
   • If it works on another network, the issue is likely with the home/primary network path (router, ISP, or DNS configuration) causing the web app to treat the traffic as blocked.
2. Test name resolution and connectivity
   • From a PC, open a command prompt and run:
     • nslookup for the target site (similar to nslookup <backend-app-name>.azurewebsites.net in the context) to see what IP is returned.
     • curl or a browser request to confirm that the 403 is returned consistently.
   • If the pattern matches the context (public IP with 403 Forbidden), it indicates that the endpoint being reached is not allowing public access from that path.
3. Use InPrivate/Incognito and clear browser data
   • Open an InPrivate/Incognito window in the browser and try to sign in.
   • If it works there, clear the browser cache and cookies and try again in a normal window.
4. If the issue persists
   • Because the error indicates the web app itself is blocking access, and the behavior is consistent for multiple users and devices, the next step is to contact the service or tenant administrator or Microsoft 365 support to check whether any network restrictions, conditional access, or service-side blocks are in place that would cause the web app to deny access from the current network.

References:

• Tutorial: Create a secure N-tier app in Azure App Service