RHEL 6 (and CentOS) have moved from sendmail to postfix. This is for the most part a Good Thing; sendmail was a mess. However it means I have to learn some new stuff. Specifically, how to convince postfix to relay email through my ISP's SMTP server.
First, I have a VM that does relaying for all my other computers. On this VM, I set up postfix to relay all mail :
# /etc/postfix/main.cf myorigin = awale.qc.ca relayhost = smtp.cgocable.ca inet_interfaces = all mynetworks = 10.0.0.0/24, 127.0.0.0/8
myorigin means user@localhost becomes firstname.lastname@example.org.
relayhost is where email is relayed to.
inet_interfaces means postfix will listen for SMTP on all the VMs networks (default is only localhost).
mynetworks means postfix will trust any email coming from a host on my LAN. Yes, this is not very secure. But I trust my LAN explicitly. I have Wifi on a separate subnet, so anything on my LAN will have to be physically connected to my LAN.
On other computers/VMs, I just need
relayhost. This last points to the postfix VM, not COGECO.