In Exchange Server 2007 (and Exchange 2010), recipients are managed from the Exchange Management Console (EMC) or Exchange Management Shell (aka the Shell). EMC does not have a Security tab for recipients.
You can still use Active Directory Users & Computers (ADUC) console to modify permissions on a recipient, as the documentation suggests ["How to Grant Send As Permissions for a Mailbox"] – and in this case, assign the Send As permission. Select the recipient in ADUC > select Properties > select the Security tab.
You can also use the Exchange Shell to quickly assign the SendAs right (for example, on a Distribution Group) to a user:
Add-AdPermission “Group Name” -user “User Name” -AccessRights
extendedright -ExtendedRights “send as”
Exchange Server 2007 SP1 includes a Manage Send As Permissions wizard, similar to the Full Mailbox Access wizard found in the RTM version.
In EMC, right-click the recipient for which you want to manage Send As permission > select Manage Send As Permission
In the Manage Send As Permission wizard, click the Add button to add the user that you want to assign the Send As permission to.