TL;DR
- Active Bug: Microsoft is investigating a Classic Outlook bug that blocks some users from sending email through Outlook.com, returning a permission-denied error.
- Root Cause: The error occurs when an Outlook.com account shares a profile with a Microsoft 365 account that has a conflicting Exchange Online mail contact.
- Workarounds: Microsoft published four workarounds, including using the Global Address List, removing the conflicting address book, or creating an isolated Outlook profile.
- Broader Pattern: The bug is one of three unresolved Classic Outlook issues in March 2026, part of an ongoing series of problems since January 2026.
Microsoft is investigating a Classic Outlook bug, disclosed in a support article updated March 31, 2026, that blocks some users from sending emails through their Outlook.com accounts. Instead of delivering messages, Classic Outlook returns a permission-denied error and generates a non-delivery report. Receiving email remains unaffected; only outgoing messages fail.
One of three unresolved Classic Outlook issues Microsoft is currently tracking, this bug joins a known issues page for March 2026 that lists eight problems for the month alone. Three remain under investigation, and five have been resolved. Microsoft described the sending error in a support article marked as under investigation with no published fix timeline. Affected products include Outlook for Microsoft 365 and Outlook.com.
What Triggers the Bug and How to Work Around It
Outlook.com accounts in an Outlook profile with another Exchange account are particularly likely to encounter this problem. According to Microsoft, a conflict arises when an Outlook.com account shares a profile with a Microsoft 365 account containing an Exchange Online mail contact that uses the same SMTP address. When Classic Outlook encounters this configuration, it returns an NDR citing error code “0x80070005-0x0004dc-0x000524” and denying the user permission to send on behalf of the specified account.
A second condition can also produce the same error: having an Exchange Online mail contact with the same SMTP address as the sender’s Outlook.com account. In both cases, Classic Outlook incorrectly interprets the send request as a delegation attempt rather than a direct send.
Microsoft provides workarounds for the sending error for users who need to continue sending email while the issue is under investigation. First, users can select “Other Email Address” from the From dropdown and choose the account from the Global Address List. Second, they can remove the Microsoft 365 account’s Address Book from Outlook’s custom address book settings via Tools, Options, Custom.
Third, administrators can hide the problematic contact from the Microsoft 365 Global Address List through the Exchange Admin Portal. A fourth option is to create a new Classic Outlook profile containing only the affected account, isolating it from the conflicting Exchange configuration entirely. The range of workarounds reflects the complexity of the underlying conflict: users and administrators face different remediation paths depending on whether they control the Exchange environment or only their local Outlook profile.
A Pattern of Classic Outlook Issues
The email delivery bug is not an isolated incident. Microsoft’s known issues page for March 2026 documents a busy month of fixes and ongoing investigations. Among recent patches, Microsoft resolved a crash that occurred when enabling the Teams Meeting Add-in and fixed Gmail and Yahoo synchronization errors that produced 0x800CCC0F and 0x80070057 error codes.
Two other bugs remain under active investigation alongside the email delivery issue: a “Can’t connect to the server” group error in Classic Outlook when Exchange Web Services is enabled, and a disappearing mouse pointer affecting Classic Outlook, OneNote, and other Microsoft 365 apps.
Classic Outlook’s bug pattern stretches back to January 2026, when Microsoft acknowledged freezing issues after the KB5074109 security update. Since then, the legacy client has seen a steady stream of new problems reported and patched on a near-monthly basis throughout the year, spanning crashes, synchronization failures, and now email delivery errors. In March 2026, Microsoft pushed the enterprise opt-out phase for New Outlook from April 2026 to March 2027, extending Classic Outlook support through 2029, while automatic migration of personal Microsoft accounts and some businesses to the new Outlook app continued throughout 2025.
Despite the extended support timeline, Microsoft has not committed to a fix date for the email delivery bug. For users blocked from sending email today, that means the four workarounds remain the only available recourse, with no patch date to plan around. Organizations still running Classic Outlook in hybrid configurations face an immediate choice: apply the profile or address book fixes, or accelerate adoption of New Outlook and Outlook on the web. With three bugs still open and Microsoft’s own workaround list pointing toward its newer product, the case for staying on Classic Outlook grows harder to make with each new issue.