Microsoft has resolved a known issue that was preventing some Classic Outlook users from sending emails via Outlook.com.
As the company explained when it acknowledged the issue last week, affected users were being warned that some of their messages hadn’t reached intended recipients.
Microsoft said that those experiencing this issue would encounter it more often when the Outlook.com account they used to send emails was an Outlook profile linked to another Exchange account.
Affected users were receiving non-delivery reports (NDRs) with 0x80070005-0x0004dc-0x000524 errors, warning that “This message could not be sent. Try sending the message again later or contact your network administrator,” when sending or replying to emails.
Microsoft also added that these errors may also be triggered when the sender’s account has an Exchange Online mail contact with the same SMTP address.
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On Friday, the Outlook Team updated the original support document, saying that the known issue has now been fixed with a server-side fix.
“The Outlook Team implemented a service change to address this issue that is in production as of April 3, 2026,” Microsoft said.
Microsoft advises those who are still affected by these email delivery issues to use the New Outlook client or Outlook.com on the web to send emails from the affected account. Another temporary workaround is to download the Outlook Address Book for any affected Outlook.com accounts, following the procedure detailed here.
Last month, Microsoft resolved another known Classic Outlook bug that rendered the email client unusable for users who enabled the Microsoft Teams Meeting Add-in.
Microsoft is also investigating a bug that triggers “Can’t connect to the server” errors when creating groups in Classic Outlook, and another issue that makes the mouse pointer disappear for some users in Classic Outlook, OneNote, and other Microsoft 365 apps.
Additionally, it’s working to resolve Exchange Online issues that have intermittently prevented Outlook mobile and macOS users from accessing their mail for weeks.
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