To Size Inbound and Outbound MTAs

In general, separate your MTA services into inbound and outbound services. You can then size each in a similar fashion. The goal of sizing your MTAs is to determine the maximum number of messages that can be relayed per second.

To size inbound MTAs, you need to know the raw performance of your inbound MTA in a real-world environment.

1. From the raw performance of the inbound MTA, add SSL, virus scanning processes, and other extraordinary message processing.

2. Account for denial of service attacks at peak volume in the day.

3. Add enough MTAs for load balancing and for redundancy as appropriate.

   With redundancy, one or more of each type of machine can still handle peak load without a substantial impact to throughput or response time.

   In addition, sufficient disk capacity for network problems or non-functioning remote MTAs must be calculated for transient messages.