Microsoft has partially mitigated a critical Exchange Online outage that caused email delivery delays and failures over the past week. The issue, tracked as EX1027675 in the Microsoft 365 Admin Center, began on March 7 and affected users worldwide.

Customers reported receiving Non-Delivery Reports (NDRs) with “554 5.6.0 Corrupt message content” errors when attempting to send or receive emails. According to Microsoft, the problem stemmed from “a recent service update intended to improve message transport services” that introduced a code issue affecting a portion of their infrastructure.

During the outage, users were unable to send email messages with attachments through Exchange Online. As a temporary workaround, Microsoft noted that sending attachments as ZIP files allowed messages to be delivered successfully.

While Microsoft has resolved the initial incident, a separate but similar issue (tracked as EX1030895) continues to affect a “small subset of messages,” causing NDR failures and problems with plain text calendar invites containing winmail.dat attachments. The company is currently testing a potential fix on isolated infrastructure and conducting targeted machine restarts to resolve the ongoing problem.

This incident follows several recent Microsoft 365 service disruptions, including authentication issues with Outlook and Exchange Online, calendar access problems for iOS mail app users, and Teams call failures—all occurring within the past few weeks.