Outlook Classic, the issue with unsent emails resolved: Microsoft releases the fix

Microsoft has confirmed the resolution of a troublesome technical issue that, in recent weeks, has prevented a significant portion of Outlook Classic users from properly sending messages through Outlook.com accounts. The bug manifested itself with the systematic receipt of non-delivery reports (NDR) containing the error string 0x80070005-0x0004dc-0x000524. According to the development team, the issue did not reside in the locally installed software on users' machines, but in incorrect handling of communications between the client and the cloud infrastructure.

The trigger was identified in specific configurations, where a profile of Outlook.com was linked to another primary Exchange account. This scenario created an identity conflict during the authentication and sending (SMTP) phase, especially if the sender's account had an email contact in Exchange Online with the same address. The result was an immediate communication block, accompanied by a text message prompting the user to try again later or contact the network administrator.

Server-side fix for Outlook Classic: error 0x80070005 resolved

Unlike other recent issues that required manual installation of new Builds, in this case Microsoft opted for a direct service change. The modification went into production on April 3 and theoretically does not require any intervention from the end user, as the fix occurs at the server infrastructure level. Despite the automation of the process, telemetry indicates that some clients may still experience residual difficulties due to the local cache of contact information or the address book.

For those still facing sending failures, the official documentation suggests a manual recovery procedure for the Outlook Address Book. Redownloading the offline address book for the affected Outlook.com account forces the client to recognize the new server directives, eliminating the old SMTP associations that caused the conflict. Alternatively, the temporary use of the web version of Outlook or the 'New Outlook' client ensures overcoming the blockage, as these interfaces do not share the legacy components of the Outlook Classic synchronization engine.

The list of known bugs in Outlook Classic is not yet exhausted, as Redmond engineers are actively investigating another anomaly that prevents the creation of groups in the service, returning the error "Unable to connect to the server." Equally unusual is the bug that causes the mouse pointer to disappear within Outlook, OneNote, and other applications in the 365 suite, an interface problem seemingly related to hardware acceleration management. Moreover, intermittent issues remain on Exchange Online affecting mainly mobile and macOS versions, with users reporting the inability to access their mailbox for several weeks. A specific resolution date for these latter issues has yet to be provided.