What is the maximum number of recipients I can message using Outlook?
The default number of allowed recipients in Office 365 is 500. For accounts that need to send larger batches of emails, we now have the ability to raise the limit and send to 1000 recipients. A ticket would need to be put in to request this recipient limit change. This is regardless of whether the addresses are listed individually, or in a distribution list you've created. There is also a hard limit of only being able to mail 10,000 recipients a day.
This being said, if you need to send to groups of students who are not in premade Outlook address book lists, you can only send to 999 at a time and must create separate lists for each group of 999 to send to and it must be sent from O365.
Note: Outlook Address book pre-defined groups (IT All Faculty, IT All Staff, IT All Students and Class lists) count as one recipient. User defined groups created in Outlook or lists of individual email addresses count as one recipient for each email address.
So to recap: you can have 1000 total recipients, including yourself. A recipient is an individual address, each address in a distribution list you create, an Active Directory Group address, or a Class List. In addition, there is a limit of 35 Class Lists allowed per email.