Use of SSL
The number of messages/connections inbound and outbound
The size of messages
The number of target destinations/messages
The speed and latency of connections to and from the MTA
The need to do spam or virus filtering
The use of Sieve rules and the need to do other message parsing (like use of the conversion channel)
The MTA is both CPU and I/O intensive. The MTA reads from and writes to two different directories: the queue directory and the logging directory. For a small host (four processors or less) functioning as an MTA, you do not need to separate these directories on different file systems. The queue directory is written to synchronously with fairly large writes. The logging directory is a series of smaller asynchronous and sequential writes. On systems that experience high traffic, consider separating these two directories onto two different file systems.
In most cases, you will want to plan for redundancy in the MTA in the disk subsystem to avoid permanent loss of mail in the event of a spindle failure. (A spindle failure is by far the single most likely hardware failure.) This implies that either an external disk array or a system with many internal spindles is optimal.
There are trade-offs between using external hardware RAID controller devices and using JBOD arrays with software mirroring. The JBOD approach is sometimes less expensive in terms of hardware purchase but always requires more rack space and power. The JBOD approach also marginally decreases server performance, because of the cost of doing the mirroring in software, and usually implies a higher maintenance cost. Software RAID5 has such an impact on performance that it is not a viable alternative. For these reasons, use RAID5 caching controller arrays if RAID5 is preferred.
The MTA does scale linearly beyond eight processors, and like the Message Store, more than linearly from one processor to four.
It is rarely advisable to put the MTA under HA control, but there are exceptional circumstances where this is warranted. If you have a requirement that mail delivery happens in a short, specified time frame, even in the event of hardware failure, then the MTA must be put under HA software control. In most environments, simply increase the number of MTAs that are available by one or more over the peak load requirement. This ensures that proper traffic flow can occur even with a single MTA failure, or in very large environments, when multiple MTAs are offline for some reason.
In addition, with respect to placement of MTAs, you should always deploy the MTA inside your firewall.