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I run cron jobs on my Mac laptop, and so I want the mail daemon to be able to send the output mail to my regular mail account. I've got that working (my local account has a .forward file), but I'm often on networks where connecting to the destination's MX on port 25 doesn't succeed.

Therefore, I would like to have the local mail daemon send its outgoing mail, regardless of the destination, to another SMTP server (which happens to be a SSH port-forward to a server under my control). How do I configure the Apple-supplied Postfix to do this?

Update:

I tried setting relayhost = [localhost]:40125 (where 40125 is the SSH port forward), but while I can now send mail to a proper Internet mail address (e.g. kpreid@switchb.org), cron job mail (which is addressed to kpreid@<whatever the DHCP hostname is>) doesn't get delivered. From the one bounce I saw, it appears that this causes all mail, even for the local hostname, to go to the relayhost first.

Therefore, I need a way to relay to [localhost]:40125 unless the address domain is this machine's domain (which varies frequently).

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You need to set the relayhost directive in /etc/postfix/main.cf (and maybe tweak some other settings, but that's hard to say without knowing the default config from Apple).

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It worked; no additional configuration was needed. I added relayhost = [localhost]:40125, where 40125 is the SSH port forward (-L localhost:40125:localhost:25); according to the comments in the file, the square brackets specify to connect directly rather than looking up a MX for the domain (which might be unnecessary for localhost, but hey). – Kevin Reid Aug 22 '10 at 12:46
    
Unfortunately, while this works for general outgoing mail, this seems to cause cron-job mail to fail because it's trying to send to "<account>@<whatever my DHCP-generated hostname is>" via the relayhost (at least, I received one bounce for this), rather than first finding the .forward in the local account. Any suggestions? – Kevin Reid Aug 28 '10 at 21:19
    
I suppose the mydestination setting isn't correct then, or maybe one of the other my* settings. You can find an explanation of all available settings in man 5 postconf. Also, on Ubuntu (I'm not sure if Mac OS X uses "Vixie cron" too?) it's possible to tell cron who to mail by setting a MAILTO environment variable in crontab (see man 5 crontab). – JanC Aug 29 '10 at 16:32

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