Dovecot

Dovecot is an open-source IMAP and POP3 email server focused on security, performance, and standards compliance. It is the most widely used IMAP server in the hosting industry and is the default mail delivery agent on cPanel, Plesk, and most other hosting platforms.

When hosting customers check their email through Outlook, Thunderbird, mobile email apps, or webmail clients like Roundcube, Dovecot is the server that handles the connection. It manages the mail store (where emails are saved on disk), handles authentication, serves mailbox contents to email clients, and manages folder structures.

Dovecot supports both Maildir and mbox mail storage formats, with Maildir being the standard for hosting due to its superior performance with large mailboxes and better crash recovery. The full-text search engine (using Solr or its built-in FTS) allows fast email search across large mailboxes.

Security features include strong SSL/TLS support, multiple authentication mechanisms (PLAIN, LOGIN, CRAM-MD5, DIGEST-MD5, etc.), per-user encryption of stored mail, and quota management. The quota system enforces disk space limits per mailbox, which is essential for shared hosting where email storage is limited.

Dovecot's LMTP (Local Mail Transfer Protocol) service acts as the bridge between Postfix (which receives incoming email) and the mail store (where it is saved). This integration is automatic on hosting panels and handles the handoff seamlessly.

The sieve plugin supports server-side email filtering rules. Customers can create rules to automatically sort incoming email into folders, forward messages, send vacation auto-replies, and reject spam based on custom criteria. ManageSieve protocol support allows email clients to manage these rules remotely.