You would like to know if Symantec Mail Security for Exchange (SMSMSE) can apply any restrictions on the number of recipients in an Email.
For example, you want to restrict a certain user to only be able to send a single Email to a maximum of 100 recipients.
Symantec Mail Security for Exchange does not have any mechanisms to control the maximum number of recipients for an individual Email, as such mechanisms are commonly available in Microsoft Exchange itself.
Please consult your Exchange documentation for further information on how this can be achieved.
At the time of writing, the following Microsoft articles discuss the setup of Recipient restrictions further: