If you want a mailbox to just receive messages, but not be able to send them:
1. Use the Microsoft Exchange Administrator program to select the mailbox.
2. On the File menu, choose Properties / Advanced Property Page.
3. Set Max Outgoing Size to 0 in the Message Sizes section.
4. Set Prohibit Send to 0 in the Information Store storage limits.
If a user tries to send a message, they receive:
The item could not be sent. You have exceeded the storage limit on your mailbox. Delete some mail from your mailbox
or contact your system administrator to adjust your storage limit.