How to send email having more than one receiver to *.protection.outlook.com smtp server(Too many recipients (AS780090)) with several RCPT_TO commands?

When you encounter the "Too many recipients (AS780090)" error while sending an email with multiple recipients to an SMTP server like '*.protection.outlook.com', it generally relates to restrictions or policies set by the server to mitigate spam and other security risks. This isn't an inherent limitation of the SMTP protocol itself, as SMTP does allow sending to multiple recipients, as outlined in the RFC 5321 document.

Here are some potential reasons for the issue and suggestions on how to address it:

1. Anti-Spam Measures: Servers like '*.protection.outlook.com' may have strict anti-spam policies. Sending emails to multiple recipients can sometimes trigger these filters, especially if the server has been configured to be particularly sensitive to potential spam activities.
2. Recipient Limits: Some SMTP servers set a limit on the number of recipients per email as a policy to prevent spam and mass-mailing worms. This limit can vary based on the server's configuration and the type of account you are using.
3. Policy of the Organization: The organization using '*.protection.outlook.com' might have set their own policies regarding the maximum number of recipients allowed for a single email. These policies are often more restrictive than the defaults provided by the service.
4. Troubleshooting Steps:
   • Split the Email: Try splitting your email into multiple messages, each with fewer recipients, to see if that bypasses the restriction.
   • Check Documentation: Review the documentation or policy guidelines of '*.protection.outlook.com' or consult with the IT department of the organization to understand their specific restrictions and policies.
   • Contact Support: If the issue persists, consider contacting Microsoft support for more detailed guidance, especially if you believe your usage is reasonable and not spam-like.
5. Review Your Sending Practices: Ensure that your method of sending emails aligns with best practices for email sending, especially in a business or organizational context. This includes not only the number of recipients but also the content and frequency of the emails.

The error message you're encountering is likely a protective measure rather than a technical limitation of the SMTP protocol. Adjusting your approach based on the specific policies and restrictions of the SMTP server you're using should help resolve this issue.

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