Hi Paul, thank you for your reply.
It will not work in this case... the server is on the internet, there is no LAN. I could disable port 587, but if someone configured Outlook to use port 25, it would work.
I also have a security gateway, but this is not a matter of security. My client simply does not want anyone (any employee) sending e-mails throught cellphones, e-mail clients, etc. It should be 100% through webmail.
The only way is to have a feature on Smartermail to disable users from sending e-mails from e-mail clients, like when we disable POP and IMAP access.
I don't believe such a feature exists, right?