How to set multiple outbound relay hosts for failover

We have set up 2 Mailsweeper server as the mail gateway for inbound and outbound internet mails. Our Domino SMTP server is suppose to route Outbound mails via these servers as a relay host.  We can set both mailsweeper server for failover for inbound internet mails via MX preferences. Can we do the same for Outbound mails?  But there is only one value that we can put in the internet outbound relay host when we have 2 relay hosts, so how do we make it to dynamically fail over if one of the mailsweeper mail gateway is down?
kamarAsked:
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qwaleteeConnect With a Mentor Commented:
You could try to load balance by assigning a round-robin FQHN in DNS, followed by the actual IPs.  It would try the FQHN first, which round-robins to load balance.

Say you have 1.2.3.4 and 5.6.7.8
Set up smtp.kamar with two A records pointing to those two addresses
Set up the relay to be: smtp.kamar,1.2.3.4,5.6.7.8

Domino server should try the FQHN first every time.  Since it resolves to altrenating addresses, it will alternate servers (unless the Domino server caches the lookup).

If 1.2.3.4 goes down, one of two things will happen on every send:
    kamar.smtp will resolve to 1.2.3.4, which is down; Domino tries it and fails.
    Domino next tries 1.2.3.4 (second relay listed), which is identical, and fails.
    Domino tries a third time with the third entry, 5.6.7.8. and succeeds
-or-
    kamar.smtp will resolve to 5.6.7.8, which is up; Domino tries it first and succeeds

If 5.6.7.8 is down, two similar possibilities occur, except that Domino succeeds on the first or second try instead of the first or third try (nevause the second try is the server that is up, whereas above, the second try is the server that is down).  So, always list your most reliable server second.
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qwaleteeCommented:
No, Domino does not use MX lookups for outbound relay resolution, so the best you can get is DNS round robin.  To do what you want, you would need a hardware load balancer, which will provide a single VIP for Domino to relay to outbound, and the load balancer will try to hit each of the two real Mailsweeper IP addresses (it sort of works like NAT, excpet that both "hidden" addresses can receive the traffic).

As a cheaper, manual alternative, assign a special FQHN for outbound relay, and manually change DNS when needed.  Or, instead of changing DNS when needed, you can pull the plug on one Mailsweeper box, and set the other to use both IP addresses during a afilure.  (Don't forget to clear ARP cache on your router when you do this).
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kamarAuthor Commented:
Someone sent a tip that we can actually specify multiple relay hosts in the configuration document

http://whatis.techtarget.com/tip/1%2C289483%2Csid4_gci765128%2C00.html

Can anyone confirm that this is workable?

Also, could you give me a bit more details on setting up the hardware load balance?
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qwaleteeCommented:
Yes, that does provide load balancing, but I have never been able to verify whether it provides failover.
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kamarAuthor Commented:
I managed to set up the multiple relay in the configuration document and tested. Two IPs separated by comma.  It does not provide load balance such that outbound smtp mails will always be sent to the first host listed in the field.  The second is only invoked if the first fails to respond.
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