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 per user per day limit is enforced by Exchange and applies to all traffic (internal and external). It is enforced by a hidden “counter” stored as a property in the user’s Exchange mailbox (Exchange throttling controls). Once this reaches the maximum number of recipients, Exchange won’t let the user send any more mail until 24 hours after the user sent the first mail counting against the recipient rate limits (see example below) . The limit is separate from (and more restrictive than) limits in FOPE.
Each Exchange Online mailbox can send messages to a maximum of 10,000 recipients per day in the Enterprise plans (E) and 10,000 per day in the Small Business and Professionals plan (P1). (in December 2011 the P1 was increased from 500 to 1,500, and in November 2012 both plans was increased from 1,500 to 10,000).
Example: An E plan user sends a mail to 500 recipients at 09:00, another mail to 500 recipients at 10:00 and yet another mail to 500 recipients at 11:00, hitting the limit of 1,500. The user will be able to send mails again 09:00 the next day (for more detailed examples see this help topic.)
So you might wonder: if you send 10 messages to the same person (over the course of a rolling 24 hour window), does that count as 10 recipients for the purpose of these limits? Or just 1?
The answer is, that it actually counts as 10 recipients (for the purposes of these limits). Each time you send a message, Exchange Online counts each recipient and adds this to your total. The fact that you sent a message to the same recipient earlier the same day doesn’t make any difference.