Here's a tip from one of the sessions I gave at ADMIN2006. We all know that to allow the best performance for mail routing, you want to have multiple mail.boxes, and that usually means setting the number to 2 mail.boxes. The question then becomes, how do you determine when it's time to increase the number of mail.boxes to increase performance again, other than by “feel.”
There are two statistics that we'll want to look at which provide the values for a calculation, but in order to monitor them, you'll need to already have enabled at least two mail.boxes as the statistics don't get created with only one mail.box. In other words, this tip won't help you determine if you need to move from one mail.box to two. The statistics to look at are:
We're going to look at the percentage of conflicts and if it is over 2%, then an increase in the number of mail.boxes may be beneficial. Add it, and monitor the stats for a period of time.
So, here's the formula to determine the percentage:
(Mail.Mailbox.AccessConflicts / Mail.Mailbox.Accesses) * 100 = Percentage of conflicts
More information on this can be found in the Determining the Number Of Mailboxes Required For a Server Lotus Technote 1148438.
⚠ UPDATE: It's important to note that you want to look at these stats over time and not let one snapshot force a change. If a stat is either over or under for a period of say a week or so, then you'll want to look at an adjustment. I didn't make that clear in the original post.