sendmail, 4th Edition

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. ^[1] 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.

^[1] One of the authors has witnessed a BSDI box delivering 1,000 messages per minute while under load averages of 200 and above.

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.