How can I let a user send on behalf of a distribution list/group?

After you create a distribution list, from Exchange Administrator, highlight the distribution list, and go to the Tools menu.

Select Options.

Select the Permissions tab. Ensure that "Show permissions page for all objects" is selected. Click OK.

Go to the Properties for the distribution list.

Select the Permissions tab, add NT accounts for the users, and assign "Send" as the role to each user you want to permit to send on behalf of the distribution list.

Select the group in the Microsoft Management Console (MMC) Active Directory Users and Computers snap-in, and then select Properties.

Select the Exchange General tab.

In the Message restrictions section, select "Only from."

Add the users. By default, all users can send to a new distribution group.