These limits are applied to messages, senders, or recipients to combat spam and mass-mailing worms or viruses.

• The maximum number of message recipients allowed in the To:, Cc:, and Bcc: fields. = 500
• The maximum number of e-mail messages that can be sent from a single e-mail client per minute = 30
  
• The maximum number of recipients that can receive e-mail messages sent from a single cloud-based mailbox in a 24 hour period = 10,000
  

Note:

1. These limits apply to both internal and external recipients.
2. For distribution groups stored in the shared address book, the group is counted as one recipient. For distribution groups stored in the Contacts folder of a mailbox, the members of the group are counted individually.

For more information from Microsoft's website, click here