Sendmail by Bryan Costales with Eric Allman et al; O'Reilly &Associates — A good Sendmail reference written with the assistance of the original creator of Delivermail and Sendmail.
Removing the Spam: Email Processing and Filtering by Geoff Mulligan; Addison-Wesley Publishing Company — A volume that looks at various methods used by email administrators using established tools, such as Sendmail and Procmail, to manage spam problems.
Internet Email Protocols: A Developer's Guide by Kevin Johnson; Addison-Wesley Publishing Company — Provides a very thorough review of major email protocols and the security they provide.
Managing IMAP by Dianna Mullet and Kevin Mullet; O'Reilly &Associates — Details the steps required to configure an IMAP server.
Security Guide; Red Hat, Inc — The Server Security chapter explains ways to secure Sendmail and other services.