The most straightforward way is to set up a system filter, and include a command such as:
unseen deliver
mailbox@whatever.domain
This sends a copy of every message to
mailbox@whatever.domain (unless the message already has that recipient - Exim never does duplicate deliveries).
To save only ``outgoing'' messages, you need to come up with a definition of what ``outgoing'' means. Typically, this might be a check on the sender address and/or on the originating host. Here is an example:
if $sender_address_domain is mydomain.com and
${mask:$sender_host_address/24} is 192.168.324.0/24
then
unseen deliver
mailbox@whatever.domain
endif
