I set a 300 recipient limit to my Exchange users at work. This is set
at my server level.
Quote from
http://office.microsoft.com/en-us/outlook/HA012002371033.aspx
Many e-mail service providers set limits for the number of names
that can be in the To, Cc, and Bcc fields in a message. For example,
your e-mail service provider may limit each message to a maximum
of 100 e-mail addresses. If these addresses can be distributed among
the To, Cc, and Bcc fields, remember that the names in the Bcc field
will count toward your total limit. Ask your e-mail service provider
about the policies for your account.
Nikki Peterson