The recipient rate limit for Office 365 Exchange Online is 1,500 recipients per day
There is a recipient rate limit, often called a sender limit or sending limit, for cloud-based e-mail accounts. This limit defines the maximum number of recipients that can receive e-mail messages sent from a single cloud-based account in a 24-hour period. The recipient rate limit for Exchange Online is 1,500 recipients per day.
The recipient rate limit applies to messages sent to recipients inside and outside your organization. After the limit has been reached, messages can’t be sent from the mailbox until the number of recipients that were sent messages in the past 24 hours drops below the limit. Multiple messages sent to the same recipient are counted separately. For example, if a user sends 10 messages to the same recipient in a 24-hour period, these 10 messages count as 10 recipients towards the recipient rate limit.
The best way to avoid exceeding the recipient rate limit is to use distribution groups or dynamic distribution groups when you send messages to large numbers of recipients. Groups are counted as a single recipient toward the recipient rate limit. For example, if you send a message to 100 recipients by adding each of those recipients on the To:, Cc:, or Bcc: fields, that counts as 100 recipients toward the recipient rate limit. But if you add those same 100 recipients to a distribution group or dynamic distribution group, and then send the same message to the group, that counts as one recipient toward the recipient rate limit.
Bulk E-Mail and Daily Recipient Rate Limits
Office 365 Exchange Online deployment