Exim

Exim is a mail transfer agent (MTA) originally developed at the University of Cambridge. It is the default MTA on cPanel servers, handling all SMTP mail delivery for the majority of the shared hosting industry. While Postfix is the default on most Linux distributions, cPanel chose Exim for its extreme configurability and tight integration with the hosting panel.

Exim's configuration language is remarkably flexible. The single exim.conf file controls every aspect of mail routing, delivery, authentication, filtering, and policy enforcement. cPanel generates and manages this configuration automatically, but administrators can customize it through the Exim Configuration Manager in WHM, which provides a safe way to add custom rules without breaking the panel's auto-generated configuration.

On cPanel servers, Exim handles incoming SMTP connections, authenticates senders (for outbound email from hosting accounts), applies spam filtering (through SpamAssassin or Rspamd), checks sender policy (SPF, DKIM, DMARC), enforces rate limits, and delivers messages to Dovecot for IMAP/POP3 access. The entire email flow from reception to mailbox delivery passes through Exim.

Exim includes powerful ACL (Access Control List) rules that execute at each stage of the SMTP transaction. These ACLs can reject spam at the connection level (before the message body is even received), verify sender addresses, check DNS blocklists, enforce authentication requirements, and apply custom policies. This pre-data rejection capability reduces server resource usage since obvious spam is rejected before transfer.

The exigrep and exim -bp commands are essential troubleshooting tools for hosting administrators. exigrep searches mail logs with message tracking (following a single email through all processing stages), while exim -bp shows the current mail queue with sender, recipients, and age.

While Exim is powerful, its complexity can be a drawback. The configuration language is essentially a programming language, and mistakes can cause mail delivery failures. For hosting environments outside cPanel, Postfix is often preferred for its simpler configuration model.