To discourage users from sending unsolicited bulk messages, Exchange Online has restrictions that prevent users and applications from sending large volumes of email. Customers who send a lot of e-mails will need to understand the limits to how many messages can be sent from a single account in a period of time in the different Office 365 plans.
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 upper limit of the recipient rate has been one of intense discussions since Office 365 first launched. This month the limit (more or less quitely) was raised from 1,500 to 10,000 emails per day (P1 as well as E plans)