Hi @J-3804
Please refer to the Sending limits in Exchange Online.
Sending limits apply to the number of recipients, number of messages, and number of recipients per message that a user can send from their Exchange Online account.
And several ways to set the limit introduced in this article: Setting Custom Recipient Limits for Exchange Online Mailboxes
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Setting a Custom Recipients Limit on a Mailbox, change it for all user mailboxes in the organization
Get-Mailbox -RecipientTypeDetails UserMailbox -ResultSize Unlimited | Set-Mailbox -RecipientLimits 10
Update Mailbox Plans for New Mailboxes
Set-MailboxPlan ExchangeOnline -RecipientLimits 50
Organization Recipient Limits in the Anti-Spam Policy
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