To enter at the General settings on the eBox mail server, go to → . At the configuration page, upper zone, some tabs will be shown to configure different things on the mail server:
Choosing services tab, we may ask for authentication to mail users or allow free access, and use (or not) TLS to cipher communications between server and client. Also, you can enable POP3 and IMAP requiring users to use SSL, just allowing them the decision or deny SSL use.
The configuration shown below this tab, allow to set a relay server, simply using its IP address. It also allows to set a maximum message limit, accepted by the server, and a default size limit for the new mail accounts. At the bottom, you can set the relay policy for each network object, which have a higher priority over the general configuration option at Services tab.
From this tab, you can enable an external filter to control eBox mail messages. To enable it, we need to set a port on eBox system to establish communications, and know IP address and port where the filter is located.