What is the maximum number of recipients I can message using Outlook?
With Microsoft 365 we now have the ability to send to 1000 recipients including yourself, so you plus 999 other addresses. This is regardless of whether the addresses are listed individually, or in a distribution list you've created.
This being said, if you need to send to groups of students who are not in premade Outlook address book lists, you can only sent 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.
The IT "All" Groups or any other group listed in the address book that are available are considered as one recipient. (ie: Class lists are considered one recipient.)
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.