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.
There are some caveats to this. If you use one of the Group addresses created in Active Directory (such as SU Henson Freshman) or one of the Class Lists (like Info 111-701 Sum1), it only counts as one address. In addition, you are only allowed to send to a maximum of 35 class lists in one email.
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.