When sendmail is installed with near-default settings, it provides excellent email services for most machines. But when installed to service high loads, high volumes, or high rates, special tuning becomes a requirement.
To understand what we mean by high in this context, consider some typical email sites on the Internet that have good delivery rates despite extremely high loads. Some can deliver email envelopes at rates exceeding 1 million envelopes per hour. Some can accept email to mailboxes for hundreds of thousands of users. Clearly, such volumes are well beyond the requirements of the average desktop workstation.
In this chapter we reveal only some of the techniques used by large sites. A full treatise on high-volume mail-handling would require a book of its own.
 One of the authors has witnessed a BSDI box delivering 1,000 messages per minute while under load averages of 200 and above.