fallback relay

[michabbs]

I want to designate one node to take care of sending mails in case of temporary problems.
So I added this to main.cf template:

[% IF dns.fqdn == 'my.other.node.com' -%]
smtp_fallback_relay = [node.responsible.for.sending.of.deferred.mails.com]:26
[%- END %]

The goal is: The 1st node tries to send an email to all applicable destination servers (i.e. "mx" or "a" servers) and if unsuccessful - it forwards it to another node for further delivery. Its seems to work. Do you think it may cause any problems?

 

[Stoiko Ivanov]

Why do you want to send the mail to a fallback? or why are the chances better for the fallback_relay to get the mail delivered?

From a quick look in the postfix documentation I don't see why this should be a problem in theory - but have not actively tried it or ran something like it in production - so keep an eye on the logs!

I hope this helps!

 

[michabbs]

Why do you want to send the mail to a fallback?

Because the node1 is physically in our office, and node2 is in remote site. We have 2 internet lines, but only one has static ip, and the other one is behind nat. In case of main line failure we still have connectivity to node2 via vpn, but are unable to deliver mails directly, because they would be refused by recipeints.

or why are the chances better for the fallback_relay to get the mail delivered?

...and node2 still has its static ip, so can deliver mails without problems.
At least that't the theory and my goal.

 

[Stoiko Ivanov]

Hmm - in this setup I probably would relay all mails to your public remote site in all cases - makes the setup a bit clearer in my opinion
also in case your static-ip line fails - how does PMG detect this situation? - I can imagine that if it still tries to deliver mails through the nat/dynamic line you might get a 5xx error from a remote server (because they simply refuse mail from dynamic ips) and then there will be no delivery to the fallback relay.
If you have some kind of mechanism that prevents this (e.g. an outbound firewall policy rejecting connections to port 25 over the dynamic line) - the setup should work as you planned.

In any case should you run into issues - just post some logs here - maybe we'll find a sensible workaround

 

[michabbs]

Hmm - in this setup I probably would relay all mails to your public remote site in all cases - makes the setup a bit clearer in my opinion

Then the remote site would become a single point of failure. In my setup both nodes can work independent and provide full in-and-out services even when the other one is down.

also in case your static-ip line fails - how does PMG detect this situation?

It does not have to detect anything special. Firewall blocks smtp via secondary line, so pmg simply gets "normal" connection refused.