What are the Office 365 email limits?

The maximum number of message recipients allowed in the To:, Cc:, and Bcc: fields is 500.  The maximum number of e-mail messages that can be sent from a single e-mail client per minute is 30. The client is identified by the user account.

The maximum number of recipients that can receive e-mail messages sent from a single cloud-based mailbox in a 24 hour period is 1,500.  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. The recipient rate limit applies to messages sent to recipients inside and outside your organization.

Additional limits are:

Message size limit 150 MB
Subject length 255 characters
File attachment limits 250 attachments
File attachment size limit – Outlook         150 MB
File attachment size limit – OWA               35 MB

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