Giving someone receive-only mail whilst not restricting access to their mail files so that mail files are still manageable by the user.
Create a group (per Server) in the Domain Name & Address Book, i.e.
Add this group to the ACL of the "ServerName" mail.box file with "No_Access". Assuming "Default" access is "Depositor" as per the standard installation.
Add users that are only to have send access to the group you have created.
Because all mail is routed through mail.box, the user will receive a Warning message stating they are not authorised "ServerName mail.box" and will be unable to send mail.
Also, because the user still has manager access to their mail files, they can delete mails to prevent mail file size issues.