Classic Outlook shipping failure with Outlook.com accounts: mitigation and what to do while Microsoft solves the problem

Several users of the classic Outlook version have started to notice that some messages are not sent correctly when using Outlook.com accounts integrated into their Outlook profile, and Microsoft already recognizes the problem and is investigating it. In practice, the persons concerned receive a non-delivery report indicating that the mail did not reach certain recipients and, in many cases, there is a permit error accompanied by the code [0x80070005-0x0004dc-0x000524], which prevents the complete shipment from the desktop client.

The failure seems to be more frequently manifested in specific configurations: for example, when the Outlook.com account is added to a classic Outlook profile that also contains another Exchange account, Outlook comes to check the Microsoft 365 tenant's address lists and, when colliding with online Exchange entries or permissions, delivery fails. Microsoft also points out that having a mail contact in Exchange Online with the same SMTP address as the shipping account can cause the same unwanted customer reaction.

While the Outlook team is looking for final correction, Microsoft has shared several temporary solutions that can help mitigate the problem without waiting for the patch. One of the suggested measures is to prevent the classic customer from consulting the Microsoft 365 tenant address book when sending messages, which reduces the checks that seem to trigger the sending refusal. Another alternative is to hide Outlook.com's contact within the tenant's Global Directory (GAL) to avoid address resolution conflicts.

If these administrative options are not feasible, there are more direct remedies at the team level: create a new classic Outlook profile that contains only the affected account, so that no account checks are mixed; or temporarily jump to the "New Outlook" client or to the Outlook.com web version to send messages from the problem account, as these environments do not seem to be affected by the same error in many cases.

For managers who want to investigate the option of hiding contacts or reviewing SMTP address conflicts, the Online Exchange documentation on how to manage mail contacts can be a useful starting point: Manage mail contacts in exchange online. And if you need to check the status of official incident services and notices, the Microsoft 365 status panel is the place where Microsoft publishes updates on known interruptions and problems: Microsoft 365 Service health.

This incident is part of a streak of problematic behaviors that the Outlook team has recently had to correct. In recent weeks Microsoft solved at least two errors that affected classic customers: one that caused unexpected closures by activating the Microsoft Teams plugin for meetings, and the other that generated error codes such as 0x800CCC0F and 0x80070057 during the synchronization of Gmail and Yahoo accounts. The company also investigates other user-reported failures, including a problem that prevents the creation of groups if Exchange Web Services (EWS) is activated and a visual failure that makes the cursor disappear in applications such as Classic Outlook or OneNote. Microsoft offers more context about one of those errors in its support center: Can't connect to the server "when creating groups (Microsoft Support).

If you are affected end user, the most practical thing right now is to try to send the mail from the Outlook.com website or from the New Outlook client and check if the problem persists. If you manage a business environment, in addition to applying the above mitigation, it is appropriate to review the Global Address List and the duplicate contact entries in Exchange Online, and maintain active monitoring of the state panel to apply the official corrections as soon as Microsoft publishes them.

It is not a temporary bad practice to move on to the web or create an isolated profile while the research continues: they are quick solutions that usually restore the ability to send emails without altering the infrastructure. Microsoft has shown transparency about the symptoms and conditions that trigger behavior, and will update its releases when there is a patch or a permanently recommended configuration change.

To follow official developments and possible step-by-step guides, the most reliable thing is to consult both the Outlook support and the channels for Microsoft 365 administrators. The Outlook help page can help resolve general questions about the service: Outlook Support. Meanwhile, maintaining good directories management practices and using alternative routes for shipping will reduce the impact on daily workflow.