Investigate recurring Exchange Online outbound spam restrictions (AS(42004)) despite unblocking the mailbox.

Hi Parvati Hatte,

The error 550 5.1.8 Access denied, bad outbound sender AS (42004) indicates that Exchange Online has restricted the mailbox from sending email because of outbound spam detection. When this happens, the mailbox can be added to the Restricted entities list. Removing the user from the list restores sending, but it does not resolve the activity that caused the restriction.

Since the restriction keeps happening after sending B2B prospecting emails, the sending pattern may be triggering outbound spam protection.

To investigate and help prevent recurrence, I recommend the following:

1. Review the outbound email volume for the affected mailbox in the Exchange admin center using the Outbound messages report. Look for unusual increases in sending volume or the number of recipients.
   
2. Check whether the mailbox is sending emails to many new external recipients within a short period.
3. Check the configured outbound spam policy. Exchange Online outbound spam policies can set limits for external recipients per hour, internal recipients per hour, and total recipients per day. For instruction how to review Outbound spam policy: Configure outbound spam policies for cloud mailboxes
   Please note that the sender must comply with both your organization's policy settings and Exchange Online sending limits.
   
4. If the mailbox continues to be restricted despite legitimate business use and under sending limit, please collect the following information and open a support case with Microsoft Support:.

• Restriction timestamps
• Outbound messages report results
• Outbound spam policy configuration
• Details from the Restricted entities page

Microsoft Support engineer can review backend and determine the specific detection responsible for the recurring restrictions.

Furthermore, I understand that prospecting emails may be important to your business. However, if you regularly need to send emails at a larger scale, you may want to consider a service designed for that type of email traffic, such as Azure Communication Services. This can help improve deliverability while reducing the risk of outbound spam restrictions on user mailboxes.

I hope these steps help you better understand the restriction and find the best way to prevent it from happening again. If you have any questions or concerns, please feel free to let me know, and I’ll be happy to assist further.