1.4 Email Server Tasks

The tasks performed by an email server running sendmail can be roughly broken down into the following categories:

1. Receive and queue email for delivery to other servers

2. Receive email and store it in the appropriate mailbox

3. Send locally generated email to other servers

The server may also run other tasks, such as allowing access to messages via POP or IMAP, but these tasks are unrelated to sendmail.

Most email servers perform each of the three aforementioned tasks, although some servers may perform one or two of these tasks far more frequently than others. The first task is representative of the workload handled by an email gateway. Large organizations often use gateways to provide a limited point of access for email to flow into and out of a network. The second task usually dominates the performance experienced by an email hub where mailboxes are stored and accessed via MUAs. The third task dominates the load experienced by an email server that handles large mailing lists or sends internally generated email out to the Internet. While many of the individual operations that constitute each of these actions remain the same from task to task, enough differences exist that this division of action becomes a convenient way to consider various aspects of email system tuning. Each of these aspects will be discussed in its own chapter in this book.