cannot send or receive emails on IOS or pc, cannot sign in on pc, and missing all emails on IOS

Dear @Chris Keen

I completely understand how frustrating this issue can be, and I want you to know that I’m here to support you. 

Based on my research, the error AADSTS900561 typically occurs when the authentication request is sent using the wrong HTTP method. Azure AD requires POST requests to process tokens and sensitive credentials, but in this case, it received a GET request instead. One of the most common causes is that your browser may be blocking third-party cookies, which are essential for the authentication flow to complete successfully. Another possibility is that your browser’s cache or cookies are outdated, which can interfere with session data and lead to unexpected behavior. Sometimes, certain browser extensions might also disrupt the authentication process. 

Therefore, here are some steps you can try: 

Step 1. Enable third-party cookies 

Could you please let us know which browser you're currently using? Here's how to allow third-party cookies for Chrome and Edge: 

Chrome:  Go to Settings > Privacy and security > Third-party cookies > Select Allow third-party cookies 

Edge: Go to Settings > Privacy, search, and services > Cookies > Turn off Block third-party cookies 

Note: If you are using other browser, please comment to let me know.

Step 2. Clear browser cache and cookies 

• Clear all cached data and cookies from your browser. 
• Alternatively, try accessing the site using Incognito/InPrivate mode. 

Step 3. Add login URLs to Trusted Sites 

• Open Internet Options > Navigate to the Security tab > Select Trusted sites 
• Add: https://login.microsoftonline.com and any other Microsoft endpoints you use 

Step 4. Try a different browser or device > This can help determine if the issue is browser specific. 

At the same time, try signing in from a private/incognito browser at Outlook on the web. If the issue continues there, it confirms that the problem is not device related.

Moreover, regarding the Outlook app in your iPhone, please try to remove the account from your iPhone. Then, use the Outlook mobile app and add it again, but this time choose Microsoft Exchange / Office 365.

If the issue sill persists, since this is a business-managed account, I recommend you contact your organization’s administrator, or whoever manages your company Microsoft 365 account. Only they can verify whether your account is still active and a Microsoft 365 license (mailbox, exchange online) is assigned. For reference: Assign or unassign licenses for users in the Microsoft 365 admin center

Note: If you do not know who your IT Admin is, please refer to How do I find my Microsoft 365 admin?

Please understand that as a forum moderator, my role is to provide general guidance and troubleshooting support. I don’t have access to internal systems or backend tools, and I appreciate your understanding of these limits.

Please try the steps above and let me know if they work. If not, we can continue working together to narrow this down.

The first response may not always resolve the issue right away, but with your help and more details, we can work toward a solution.

Thank you for your patience and understanding. I'm looking forward for your reply.

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