Microsoft Fixes Classic Outlook Bug Causing Email Delivery Issues
Moderate risk — monitor and plan remediation
Basically, Microsoft fixed a problem that stopped some users from sending emails.
Microsoft has fixed a bug causing email delivery issues for Classic Outlook users. This problem led to non-delivery reports for some emails. Affected users should switch to the New Outlook client or Outlook.com for sending emails.
What Happened
Microsoft has recently addressed a bug in Classic Outlook that was preventing some users from successfully sending emails via Outlook.com. This issue led to users receiving non-delivery reports (NDRs) with specific error codes, indicating that their messages had not reached the intended recipients. The problem was particularly prevalent for users whose Outlook accounts were linked to other Exchange accounts.
Who's Affected
The bug affected users of Classic Outlook who were trying to send emails from accounts linked to Exchange accounts. Those encountering the issue received error messages like "This message could not be sent. Try sending the message again later or contact your network administrator." This issue was reported to occur more frequently when the sender's account had an Exchange Online mail contact with the same SMTP address.
What Data Was Exposed
While the bug primarily impacted email delivery, it did not expose any sensitive data. However, the inability to send emails could lead to communication delays, affecting users' productivity.
What You Should Do
For those still experiencing issues, Microsoft recommends switching to the New Outlook client or using Outlook.com on the web to send emails. Additionally, a temporary workaround involves downloading the Outlook Address Book for affected accounts. Microsoft has confirmed that the issue has been fixed with a server-side update that went into production on April 3, 2026.
Additional Context
This is not the first time Microsoft has addressed issues within Classic Outlook. Recently, they resolved another bug that rendered the email client unusable for users who had enabled the Microsoft Teams Meeting Add-in. Microsoft is also investigating other bugs that affect connectivity and functionality within Classic Outlook and other Microsoft 365 applications.
By addressing these issues, Microsoft aims to improve user experience and ensure that email communication remains reliable for all users.
🔒 Pro insight: This fix highlights the ongoing challenges Microsoft faces in maintaining compatibility across its legacy and modern applications.