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The question is not directly related to iredmail but I think that this forum is the best place to post it if I am searching for a reliable answer.
The configuration is as follow
1 iredmail server
2  adsl connections each with a public ip address. Adsl-1 with ip x.x.x.x and Adsl-2 with ip y.y.y.y.
A port forward rule to allow incoming smtp traffic - from publi ip x.x.x.x to the private ip of iredmail and a rule to allow outgoing  smtp traffic from iredmail to the Internet configured to use only public ip x.x.x.x.
An mx record pointing to public ip x.x.x.x.
Now in order to have some kind of failover in case Adsl-1 (x.x.x.x) goes down I thought of doing the following:
-- Add a second mx record with a lower priority pointing to Adsl-2(y.y.y.y).
-- Add a rule to allow incoming smtp traffic from public ip y.y.y.y to the private ip of iredmail.
-- In case Adsl-1 goes down manually change the outgoing smtp rule to use the second public ip (y.y.y.y).
I guess that with the above scenario I will be able to receive emails when Adsl-1 fails (normally the sending smtp server will use the second mx y.y.y.y when the first is not responding) but I have doubts about the outgoing smtp traffic - will the receiving smtp server accept my emails coming from ip y.y.y.y?
Has anyone tried a similar configuration ?
Thank you