Office 365 - Exchange Online Limitations 

A few facts about Exchange Online which may be of interest.

• Each Exchange Online mailbox can send to a maximum of 1500 recipients per day
• A single email message can be addressed to a maximum of 500 recipients per day
• Each mailbox will only send up to 30 messages per minute
• These limits apply to emails sent within an organisation, as well as externally
• Distribution groups stored in the Global Address Lists (for example, All Trident) counts as one recipient. 
• In a personal distribution group each recipient is counted separately