550 5.1.90 NDR

This should be properly documented :

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🚨 Received "550 5.1.90" when sending internal emails in Exchange Online?

Error:

Remote Server returned '550 5.1.90'
Your message can't be sent because you've reached your daily limit for message recipients. [techcommun...rosoft.com], https://learn.microsoft.com/en-us/answers/questions/5887914/ndr-550-5-1-90-daily-recipient-limit-exceeded-for

💡 Important Tip

If you're sending emails to a large number of internal recipients, consider using a Distribution List (DL) instead of individual email addresses.

Why?

✅ For the recipient rate limit, a Distribution Group stored in the organization's address book counts as a single recipient.
✅ This helps reduce the recipient count against the sender's daily limit.
✅ Personal distribution lists stored in a user's Contacts folder do not provide this benefit, as each member is counted individually. [techcommun...rosoft.com]

Best Practice

• Use organization-managed Distribution Lists for internal communications.
• Avoid sending large emails to hundreds of individual recipients.
• Review automated workflows and service accounts that send high-volume emails.

A small change in how recipients are addressed can help prevent unnecessary NDRs and improve mail flow efficiency.

#Microsoft365 #ExchangeOnline #M365 #ExchangeAdmin #ITPro #EmailAdministration #DistributionLists #TechTips #MicrosoftExchange

Daily Recipients (per 24-hour rolling window): 10,000 recipients per user 
https://learn.microsoft.com/en-us/defender-office-365/outbound-spam-sending-limits-troubleshoot

Note

For the recipient rate limit, distribution groups in the organization's address book count as one recipient. Distribution groups in a mailbox's Contacts folder are counted individually by member.

Red Flag -I am also seeking answers to the same issue. Have you found any leads from any other sources?