How to fix Remote server returned '550 5.7.233 - Your message can't be sent because your tenant has exceeded its daily limit for sending email to external recipients (tenant external recipient rate limit). For more information see https://aka.ms/EXONdrs.

Hi @Admin / Quantum Behavioral Solutions, Inc, 

I hope you are having a wonderful day today.  

Based on the details you shared that you are currently experiencing an issue where your outbound emails are being blocked in Classic Outlook with the error 550 5.7.233, indicating that your tenant has exceeded its daily limit for sending emails to external recipients. 

This error occurs because Microsoft Exchange Online enforces a Tenant External Recipient Rate Limit (TERRL), which is a daily cap on the number of external email addresses your organization can send to within a rolling 24-hour window. The limit is calculated based on the number of active email licenses assigned in your tenant. Once this threshold is reached, all outbound messages to external recipients will be temporarily blocked until the sending volume from the past 24 hours naturally drops below the limit.  

For a full overview of how this policy works, please refer to the official Microsoft article here: Introducing Exchange Online Tenant Outbound Email Limits | Microsoft Community Hub 

To help you move forward, below are some workarounds that you can take: 

1/ Allow the block to reset automatically 

• As a first course of action, please allow some time for the restriction to lift on its own.  
• Since the limit is tracked on a rolling 24-hour sliding window, the block will be released automatically once the outbound email volume from the last 24 hours falls below your tenant's threshold.  
• Depending on your organization's sending pattern, this may take anywhere from a few minutes to up to 24 hours.  
• During this time, it is strongly advised to avoid sending additional emails to external recipients, as doing so may further extend the block. 

2/ Monitor your tenant's daily email usage via the Exchange Admin Center (For Administrators) 

Once you have access to send emails again, it would be beneficial to review your organization's outbound email activity to better understand what triggered the limit.  

• To do this, please sign in to the Exchange Admin Center (EAC) 
• Go to Reports > Mail Flow > Tenant Outbound External Recipients Rate.  

This report will show your current usage, your tenant's daily limit, and whether any messages have already been blocked.  

Reference: Mail flow reports in the new EAC in Exchange Online | Microsoft Learn 

Additionally, you can run the following PowerShell command using the Exchange Online Management module (v3.7 or later) to check your tenant's threshold and current observed volume: Get-LimitsEnforcementStatus

This will help you identify whether this is a one-time spike or a recurring pattern that needs a longer-term adjustment. 

3/ Explore a long-term solution based on your sending needs 

If your organization frequently sends high volumes of emails to external recipients, there are two recommended paths forward: 

• Increase your Microsoft 365 license count: a higher number of active licenses will raise your TERRL threshold accordingly. 
• Switch to Azure Communication Services (ACS) Email for bulk or high-volume sending. ACS is Microsoft's recommended platform for this use case and operates outside of Exchange Online's sending limits, making it well-suited for newsletters, notifications, or any large-scale outbound email needs. Overview of Azure Communication Services email - An Azure Communication Services concept article | Microsoft Learn 

I hope this information is helpful. Please follow these steps and let me know if it works for you. If you have any updates regarding the issue, please feel free to share them with me.          

Thank you for your patience and your understanding. I look forward to continuing the conversation.

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