Microsoft links Classic Outlook issue to email delivery problems

Microsoft is currently addressing a known issue that affects some users of Classic Outlook, preventing them from successfully sending emails via Outlook.com.

The users impacted by this problem are encountering notifications indicating that their messages have not been delivered to their intended recipients. This issue appears to be more prevalent among Outlook.com accounts that are associated with another Exchange account.

The non-delivery report (NDR) error message that users receive reads as follows: "This message could not be sent. Try sending the message again later or contact your network administrator. You do not have the permission to send the message on behalf of the specified user. Error is [0x80070005-0x0004dc-0x000524]."

Moreover, Microsoft has identified that another factor contributing to these errors occurs when the sender's account has an Exchange Online mail contact that shares the identical SMTP address.

While the Outlook team works on a resolution for this issue, they have provided several temporary workarounds for customers affected by the problem. Microsoft suggests the following:

• Remove the M365 account from the Address Book to prevent the Outlook client from referencing it while sending emails.
• Hide the Outlook.com contact from the Global Address List (GAL) of the Microsoft 365 account.
• Create a new Classic Outlook profile that includes solely the account facing NDR errors.
• Utilize the New Outlook client or the Outlook.com web platform to send emails from the problematic account.

In recent weeks, Microsoft has resolved two additional known issues: one involving Classic Outlook crashing when enabling the Microsoft Teams Meeting Add-in, and another related to errors during the synchronization of Gmail and Yahoo accounts.

Furthermore, Microsoft is investigating other bugs that lead to "Can't connect to the server" errors when creating groups with Exchange Web Services (EWS) enabled, as well as issues that result in the mouse pointer disappearing in Classic Outlook, OneNote, and various other Microsoft 365 applications.