The following limits are applied in Office 365 to messages, senders, or recipients to combat spam and mass-mailing worms or viruses.
| Feature |
Limit |
| Recipient limit |
500 recipients |
| Recipient rate limit |
10,000 recipients per day |
| Message rate limit (SMTP client submission only) |
30 messages per minute |
Recipient Limit
This is the maximum number of recipients allowed in the To:, Cc:, and Bcc: fields for a single email message. For the purposes of the recipient limit, a distribution group that is stored in the organization's shared address book counts as one recipient. In a personal distribution list, each recipient is counted separately.
Recipient Rate Limit
To discourage the delivery of unsolicited bulk messages, Exchange Online has recipient limits that prevent users and applications from sending large volumes of email. These limits are applied per-user to all outbound and internal messages. For the purposes of the recipient rate limit, a distribution group that is stored in the organization's shared address book counts as one recipient. In a personal distribution list, each recipient is counted separately.
Message Rate Limit
Message rate limits determine how many messages a user can send within a specified period of time. This limit helps prevent overconsumption of system resources by a single sender. If a user submits messages at a rate that exceeds the limit via SMTP client submission, the messages will be rejected and the client will need to retry. For users submitting via any other mechanism, the messages are accepted, but Exchange Online will queue them at the server and throttle the rate of delivery.
Source: https://technet.microsoft.com/en-us/library/exchange-online-limits.aspx#RecipientLimits