How can i test if postfix really uses secondary mailserver if first one is not available?

Question

when i query for the mailservers of domainx with "dig domainx.com MX" the result is:

...
;; ANSWER SECTION:
domainx.com. 55188 IN MX 120 mail2.domainx.com.
domainx.com. 55188 IN MX 100 mail.domainx.com.
...

When the webserver (postfix) sends a mail to someone@domainx.com it uses mail.domainx.com as relay.

Now, i want to test if he really switches to mail2.domainx.com if mail.domainx.com is not available. I have no access to the mailservers, just the webserver which is running postfix.

I thought of temporarily making a false entry in /etc/hosts for mail.domainx.com on the webserver - will this work?

Is there another and better way?

Answer 1

You could temporarily add a blackhole route to the primary MX and then observe Postfix's behaviour.

Linux

ip route add blackhole ip.add.re.ss

Solaris/BSD

route add -host ip.add.re.ss 127.0.0.1 -blackhole

Answer 2

Usualy, I test this problematic with the firewall : I put a filter from my server to the IP of mail.domainx.com, on the SMTP port (25/tcp).

Then, Postfix can't join the first server and should try the second.

But I think you can do the same thing with your /etc/hosts proposition, by choosing an IP which doesn't exists.