The maximum number of recipients of a single email sent in Outlook is 500, and a total of 10,000 recipients in any 24 hour period from any single mailbox (details here). If you need to send an email to more than this number of recipients, you can use one of the following mechanisms:
IT Services provides a Mass Mailing application which is designed to make it easy to mail groups of staff and students at Warwick (though note that the application does not support sending email to people outside of Warwick). It can be used by any member of staff, and understands groups such as:
any group of students, based on department, course or module data
all staff and students in a department
all staff in a department
all members of any web group (you can make your own web groups by adding individuals or pre-existing web groups together, so this makes Mass Mailing very flexible)
The service is available 24/7, other than very occasional periods of downtime for planned maintenance, and is free at the point of use.