Microsoft has fixed a server-side issue that was preventing some Classic Outlook users from successfully sending emails via Outlook.com. The bug, first acknowledged last week, caused affected users to receive non-delivery reports (NDRs) with error codes such as 0x80070005-0x0004dc-0x000524, warning that messages could not be sent.
What Happened
- Symptoms: Users saw repeated NDRs when sending or replying to emails.
- Trigger conditions:
- Outlook.com accounts linked to another Exchange profile.
- Accounts with an Exchange Online mail contact using the same SMTP address.
- Error message: “This message could not be sent. Try sending the message again later or contact your network administrator.”
Resolution
- On April 3, 2026, Microsoft rolled out a service change that fixed the issue.
- The fix was applied server-side, meaning users don’t need to install updates.
- Microsoft advises:
- If problems persist, use the New Outlook client or Outlook.com on the web.
- As a temporary workaround, download the Outlook Address Book for affected accounts.
Wider Context
- March 2026: Microsoft resolved another Classic Outlook bug that broke functionality when the Teams Meeting Add-in was enabled.
- Ongoing investigations:
- “Can’t connect to the server” errors when creating groups.
- Mouse pointer disappearing in Classic Outlook, OneNote, and other Microsoft 365 apps.
- Exchange Online issues intermittently affecting Outlook mobile and macOS users.
Final Thought
This fix highlights Microsoft’s ongoing struggle to maintain stability in Classic Outlook, especially as more users transition to the New Outlook experience
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