Forum Discussion
550 5.1.90 NDR
Problem: NDR "Remote Server returned '550 5.1.90 Your message can't be sent because you've reached your daily limit for message recipients." Question 1: the code 550 5.1.90 doesn't exist under cod...
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.