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To help prevent SPAM, Exchange Online limits the number of recipients a single user can send mail to within a 24 hour period. This limit has affected some companies who need to send many emails. To address this, Exchange Online has updated the 24 hour send limit from 1,500 to 10,000 recipients.
New customers will receive the new limits. The limits for existing customers are being adjusted now and should be applied to all existing customers by the first week of December.
To learn more, see Message and Recipient Limits.