SquirrelMail

SquirrelMail is a classic, PHP‑based webmail client that has remained relevant for more than two decades. It presents a lightweight, server‑side application that interfaces with standard IMAP/SMTP mail servers, offering users an email experience comparable to desktop clients while running entirely within a web browser. From a developer’s standpoint, the project is notable for its long‑standing commitment to backward compatibility—evidenced by regular updates that bring support for the latest PHP releases (PHP 8 as of 2021, and earlier patches for PHP 5.4/5.5). This makes it a dependable choice for environments that must run legacy code alongside newer stacks.

Key Features

• Pure PHP implementation – No external binaries or native extensions are required; the entire codebase is written in PHP 5.x/7.x syntax, making it trivial to deploy on shared hosting or custom LAMP stacks.
• IMAP/SMTP integration – Uses PHP’s IMAP and SMTP extensions to fetch, send, and organize mail. Supports multiple folders, message threading, and MIME parsing.
• Plugin architecture – A modular plugin system allows developers to extend functionality (e.g., the “Mail Fetch” or “Junk Email Filter” plugins) by placing PHP files in a dedicated directory and enabling them via a simple config flag.
• User‑level configuration – Each user can customize themes, signature blocks, and mailbox layout through a web interface; settings are stored in a lightweight SQLite or MySQL database.
• Security hooks – Built‑in mechanisms for handling phishing and spam detection, with the ability to plug in external filters or API calls.

Technical Stack

The core of SquirrelMail is a collection of PHP scripts that render the UI and handle business logic. It does not rely on any heavy frameworks, which keeps the footprint small (≈ 5 MB of code) and eases static analysis or code review.

Core Capabilities & APIs

• Message API – Exposes functions such as SquirrelMail_imap_fetch_overview() and SquirrelMail_mailbox_send_message(), which developers can call directly from custom plugins or external scripts.
• Event hooks – Plugins register callbacks for events like preLogin, postLogout, or messageSent. This allows deep integration without modifying the core codebase.
• REST‑like endpoints – While not a full REST API, SquirrelMail’s AJAX handlers (e.g., ajax.php) provide JSON responses for tasks such as fetching message bodies or updating mailbox settings.
• Configuration API – The config.php file is parsed by the application; developers can programmatically update settings or generate user‑specific config files for automated deployments.

Deployment & Infrastructure

SquirrelMail is designed to run on virtually any LAMP stack. It requires PHP with the IMAP extension enabled and a writable directory for temporary files. For scalable deployments:

• Horizontal scaling – Multiple instances can be load‑balanced behind a reverse proxy; session data is stored in the database, ensuring consistency across nodes.
• Containerization – Official Docker images (or community‑maintained ones) encapsulate the PHP runtime and web server, simplifying CI/CD pipelines.
• High‑availability – By storing configuration in a shared database and using sticky sessions or distributed session stores, the application can survive node failures.

Integration & Extensibility

The plugin system is the linchpin of SquirrelMail’s extensibility. Each plugin resides in plugins/ and contains a manifest file (plugininfo.php) that declares metadata, dependencies, and event hooks. Developers can:

• Create lightweight extensions that add new menu items or modify the message view.
• Hook into IMAP operations to inject custom headers, perform analytics, or integrate with external identity providers.
• Leverage the existing “Mail Fetch” plugin as a template for writing IMAP clients that pull mail from other protocols (POP3, MBOX).

Because the code is open source under a permissive license (BSD‑2), there are no vendor lock‑ins, and the community actively maintains security patches.

Developer Experience

• Documentation – The project provides a comprehensive developer guide, API references, and plugin development tutorials. While the documentation is older in style, it covers all core concepts.
• Community – Mailing lists and forums remain active; the project hosts a bounty system for feature requests, encouraging community contributions.
• Testing – The codebase includes unit tests (PHPUnit) and integration tests for core functionality, making it easier to adopt in test‑driven workflows.

Use Cases

1. Enterprise intranet – Organizations that require a webmail client with strict control over the codebase can deploy SquirrelMail on their internal servers, integrating it with existing LDAP/Active Directory for authentication.
2. Custom mail portals – Service providers can bundle SquirrelMail with additional plugins (e.g., calendar, contact sync) to offer a turnkey webmail solution