Omdat het naar mijn mening niet echt heel helder staat beschreven in de IBM Domino documentatie een tip hoe je een SMTP failover/load balancing kunt realiseren met IBM Domino vanaf versie 6.

IBM geeft in de normale Domino Administrator Help het volgende aan:
For the field Relay host for messages leaving the local Internet domain enter the host name, domain name, or IP address of the server being used as a relay host keeping the following guidelines in mind:
A domain name is a valid entry only if the internal DNS contains an MX record for that domain and can resolve it to a host name.
When entering an IP address, enclose it within brackets; for example, [127.0.0.1].

Er is echter een technote die aangeeft dat vanaf versie 6 ook meerdere machinenamen/ip-adressen ingevoerd mogen worden:
http://www.ibm.com/support/docview.wss?uid=swg21217089
Using the Configuration Document to implement outbound SMTP failover

Question:
You must configure the Lotus Domino® server for outbound SMTP failover. In the Configuration Document –> Router/SMTP panel –> Basics panel, can the field, “Relay host for messages leaving the local internet domain,” contain multiple hostnames/IP addresses?

Answer:
This feature is available beginning with Domino 6 releases. Use a semi-colon to separate hostname entries or IP addresses to achieve failover.
Example: host1.acme.com;host2.acme.com

Result:
The router will attempt an SMTP transfer to host1.acme.com. If host1.acme.com is down or not responding, the router will failover to host2.acme.com as observed below:

09/13/2006 11:01:46 AM Router: No messages transferred to host1.acme.com;host2.acme.com (host host1.acme.com) via SMTP: The server is not responding. The server may be down or you may be experiencing network problems. Contact your system administrator if this problem persists.
09/13/2006 11:01:46 AM SMTPClient: Attempting to Connect: Host host2.acme.com, Port 25, SSL Port 0, Connecting Domain vec.lotus.com
09/13/2006 11:01:46 AM SMTPClient: Connection successful

Nu kreeg ik direct een foutmelding dat de remote server niet gevonden kon worden.
Vandaar een zoektocht naar meer informatie.

Op Slideshare vond ik de volgende IBM presentatie:

How to make Domino relay server to route mail to mutiple SMTP hosts
Within the Domino “Relay host” field, two list delimiters are allowed – comma( , ) and semicolon( 😉
● Method 1: Load balancing mode : host1, host2
In this example, the router will take the group of hosts at the
same preference level (host1 and host2) and try to connect to
these hosts in a random order.
● Method 2: Fail over mode : host1; host2
In this example, the router will take preference of first host
(host1) to route emails. If host1 stops responding, then the
Domino router will move on to the next host (host2) i.e the next
preference level.

Zie ook:
https://www.ibm.com/support/knowledgecenter/SSKTMJ_8.0.1/com.ibm.help.domino.admin.doc/DOC/H_EXAMPLE_OF_USING_TWO_SERVERS_TO_BALANCE_INTERNET_MAIL_LOAD_EX.html

The internal mail servers can route Internet mail to the server with SMTP enabled for external mail either via IBM® Lotus® Notes® routing, with a Foreign SMTP Domain document and SMTP Connection document linking to the SMTP server, or via SMTP routing, with the SMTP server configured as the relay host.

Configuring these servers requires:

• Enabling “SMTP used when sending messages outside of the local Internet domain” for Mail1 and Mail3.
• Enabling the SMTP listener task for Mail1 and Mail3.
• Setting up DNS correctly to include MX records for Mail1 and Mail3, indicating to external SMTP systems that these are the hosts that receive inbound mail for the acme.com domain.
• Either enabling “SMTP allowed outside of the local Internet domain” for the internal mail servers, Mail2 and Mail4, and listing Mail1 or Mail3 as the relay host, or creating a Foreign SMTP Domain document and SMTP Connection document that define the route to Mail1 or Mail3.

Het leek mij logisch dat als je 1 IP-adres moet noteren in blokhaken, je twee IP-adressen gescheiden door een comma of puntcomma dan als volgt zou invoeren: [10.10.1.4],[10.10.1.6]
Dit werkte echter niet.

Na weer wat zoekwerk op internet vond ik een ouder document uit 2011:
http://www.ibm.com/support/docview.wss?uid=swg21502757

Question:
Can we configure multiple IP to Domino outbound SMTP server ?
e.g.- 10 IP to send out the mails so that mail transfer will be faster by 10 times.

Answer:
You can configure multiple IP addresses in the relay host field in the Configuration Document for the Lotus Domino® server. (Configuration Document –> Router/SMTP panel –> Basics –> Relay host for messages leaving the local internet domain ).

You can use multiple IPs by separating with a comma or a semi-colon as the separator between IP addresses in the relay host field in the Configuration Document, or if using brackets then add a second bracket to the first IP address as given example below.

Note:
Don’t use brackets around or spaces within the IP address configuration.

Use:
10.10.1.4,10.10.1.6
or
10.10.1.4;10.10.1.6
or
[[10.10.1.4],[10.10.1.6]

In ons geval moest er dus een extra blokhaak voor het eerste IP-adres!

Deze load balancing methode werkt overigens op meer plaatsen volgens de laatste link:

Question:
I am using 3 gateways for out going mails is it possible that i can configure load balancing?

Answer:
You can use a comma to separate the IP addresses of your gateways. Comma will give you load balancing while semicolon will give you failover. Specify the IP addresses like this x.x.x.x, y.y.y.y, z.z.z.z .

(Weer een andere notatie, of toch met [] of [[]?)