Below are the Hard Sending Limits for Office 365:

Sending Limits	PreAU	Students	Employees	Retirees	Alumni
External Recipients (per hour)	100	1000	1000	1000	100
Internal Recipients (per hour)	100	1000	1000	1000	100
Maximum Recipient (per 24-hour period)	100	1000	1000	1000	100
Maximum Recipient (per email)	100	500	500	500	100

 

The limits listed above are total amounts for a 24-hour period. 

Example: If you were to send 800 emails at 8 pm tonight and 200 more at noon tomorrow, you would meet the limit shown above.  These numbers would reset at 8 pm.  If you go over the limit of 1,000 emails in the time period, you would not be able to send or receive emails for 24 hours from the time that you went over the limit.

A potential workaround to the 500-recipient limit per message is to do a mail merge.  In a mail merge Office 365 sends each recipient a separate message, however, the 1,000 limit per day still applies.