Microsoft Cracks Down on Spam Email with Onmicrosoft.com Domain Limitations
Microsoft has taken action against spammers who have been exploiting its onmicrosoft.com domain for their email campaigns, known as Microsoft Online Email Routing Address (MOERA) domains. The tech giant aims to curb the practice of spammers using these domains to flood inboxes with unwanted emails by creating new ones when existing ones are flagged by Microsoft’s anti-spam defenses.
From December 1, organizations utilizing the domain will be restricted to sending emails to a maximum of 100 external recipients within a 24-hour period. Initially, these restrictions will apply to smaller organizations with fewer than three seats (separate accounts registered under one organization). Microsoft plans to gradually roll out the new restrictions, with the limit applying to organizations with up to 10 tenants by January 7, 2026, and expanding to those with up to 50 seats on February 2, 2026. By June 1, 2026, the restrictions will affect organizations with over 10,001 seats.
If an organization’s emails are blocked due to exceeding this threshold, they will receive a Non-Delivery Report (NDR) message with error code 550 5.7.236. To avoid these limitations, users can opt for purchasing and migrating to a custom domain or ensure that only custom domains are used for sending external emails.
However, changing the primary SMTP address from onmicrosoft.com could lead to complications. Organizations using it as their main SMTP address would need to manage and update credentials across all devices and applications, which may include notifying clients and partners about the change. Microsoft will notify customers impacted by each stage of the rollout one month in advance via the Message Center and encourages all customers utilizing onmicrosoft.com to start planning and migrating immediately.
While cracking down on spam is generally beneficial, some concerns have been raised about the timing of the new restrictions. They coincide with the end-of-life for many versions of Windows 10, potentially burdening Microsoft administrators further.
