I'm mostly a very lightweight geek, but I do one thing that's quite hard core, these days: I run my own mailserver, rather than letting Google do it for me. (I know they just want to love me. I know, I know.)
This includes outgoing mail. I send all my mail through my own postfix server, which - until today - relayed it all through my ISP's SMTP server. Geeks have been doing this sort of thing for years.
Lately I've had a bit of a problem: more and more of my mails are getting rejected by SPF. Especially ones from my redhat.com address, because redhat.com implements SPF, and so any server which also implements it will flag or refuse mail that claims to be from a redhat.com email address but which didn't arrive from Red Hat's outgoing mail server.
This is a bit of a pain. If you're in the same situation but you don't run your own mail server, all you need to do is make sure each of your accounts is set to send outgoing mail through the correct server, don't just send them all through your ISP server. If you run your own mail server which then relays the mail, though, it's a bit trickier.
I started on a whiny post about how SPF was evil, then decided I'd better just try and fix it instead. So with a few minutes of Googling, here's how you do it, with Postfix at least. Postfix has a feature which lets you override the relayhost parameter conditionally, depending on the address the email claims to be from. So, I added a line like this in my /etc/postfix/main.cf:
sender_dependent_relayhost_maps = hash:/etc/postfix/sender_relay
and I created the file /etc/postfix/sender_relay , which contains this single line:
et voila. Any mail sent through my server which claims to be from email@example.com gets relayed via Red Hat's SMTP server (no, that's not its real address), not via my ISP's server (which all my other mail gets relayed through). That was simple.