IBM Domino HowTo: Determining the number of mailboxes required for server and set smtp relay as failover or loadbalancing

During last days I was setting some things on domino servers so i like to recap 2 basic topic that I know is not so popular as they must be inside Admin community

Determining the number of mailboxes

Here I suggest to start with this basic rule: if a Domino server is a mail server start with 2 mailbox.
If you like to know if your environment could be run better with more mailboxes you have to make this expression:

(Mail.Mailbox.AccessConflicts / Mail.Mailbox.Accesses) x 100  

If the result is greater than 2 you have to add a mailbox. 

To obtain the 2 stats use the following command on Domino console

show stat mail

Here the descriptions of the stats

Mail.Mailbox.Accesses	Total number of times that threads accessed any mailbox on the server.
Mail.Mailbox.AccessConflicts	The number of times that a thread attempting to access a mailbox had to wait because the number of concurrent threads exceeded the number of mailboxes configured. For example, if there are three mailboxes configured, and there are four concurrent accesses, the conflict count would be incremented. If the number of access conflicts consistently exceeds two percent of the value of Mail.Mailbox.Accesses, consider creating an additional mailbox.

Read this technote to have more detail

SMTP Mail relay - Load Balancing / Fail Over

If you have to use an SMTP relay from a Domino server you have to go on config document of the server on  tab Router/SMTP --> Basics and set the value Relay host for messages leaving the local internet domain.

Here you can use comma or semicolon as a separator between the hosts
eg

[10.1.2.3],[10.1.2.4]   here we have a load balancing using random access

[10.1.2.3];[10.1.2.4]   here we have failover between the first one and than the second one

 Check this slides  to have more detail about this topic and SMTP in general