WildDuck Server

WildDuck is the core component of the Zone Mail Suite. It provides IMAP and POP3 access to email, a comprehensive HTTP API for management, and handles all email storage in MongoDB.

Services

WildDuck runs the following services (each can be independently enabled/disabled):

Service	Default Port	Protocol	Purpose
IMAP	993 (TLS)	IMAP4rev1	Email client access
POP3	995 (TLS)	POP3	Email client access
HTTP API	8080	REST/JSON	Management, webmail backend

All services support TLS with SNI (Server Name Indication) for hosting multiple domains with different certificates.

Stateless Design

WildDuck instances are completely stateless — all session state is stored in MongoDB and Redis. This means:

• Multiple WildDuck instances can run simultaneously
• Users can connect to any instance (no sticky sessions needed)
• Instances can be added or removed without disruption
• A TCP load balancer (e.g., HAProxy) distributes connections

Session state is managed as follows:

• When a mailbox is opened, the current UID list is loaded from MongoDB
• Changes (new/deleted messages, flag updates) are tracked in a journal collection
• Redis pub/sub propagates change notifications between instances
• Each instance applies journal entries to keep its view of the mailbox current

MongoDB Storage Model

WildDuck uses MongoDB as its sole persistent storage backend. Key collections:

Core Collections

• users — User accounts, passwords, quotas, settings
• addresses — Email addresses mapped to users (including wildcards)
• mailboxes — Mailbox hierarchy per user (up to 1500 mailboxes, 128 levels deep)
• messages — Message metadata, parsed MIME tree, headers, flags
• threads — Message threading information
• journal — Write-ahead log for IMAP state changes

Security Collections

• authlog — Authentication attempt log (TTL-indexed, 30 days)
• asps — Application-specific passwords
• audits — Audit records

System Collections

• settings — System-wide configuration values
• tasks — Background task queue
• dkim — DKIM signing keys
• certs — TLS certificates
• webhooks — Webhook subscriptions
• domainaliases — Domain alias mappings
• domainaccess — Domain allowlists/blocklists

Attachment Storage (separate database)

• attachments.files / attachments.chunks — Deduplicated message attachments (GridFS)
• storage.files / storage.chunks — Draft file uploads (GridFS)
• audit.files / audit.chunks — Audit message copies (GridFS)

Background Tasks

WildDuck includes a task queue system backed by MongoDB and Redis distributed locking. Task types include:

• user-delete — Asynchronous user account deletion with full data cleanup
• quota — Quota recalculation
• acme / acme-update — ACME certificate provisioning and renewal
• clear-folder — Bulk folder clearing
• search-apply — Apply filters to existing messages
• user-indexing — ElasticSearch indexing for a user
• restore — User data restoration
• run-migrations — Database schema migrations

ElasticSearch Integration

WildDuck optionally supports ElasticSearch for full-text message search. When enabled:

• New messages are indexed in real-time via MongoDB change streams
• Historical messages can be bulk-indexed
• Search queries use ElasticSearch instead of MongoDB text indexes
• The feature is gated per-user via a Redis set (feature:indexing)

Key Limits

Limit	Default Value
Max message size	64 MB
Max attachment size	25 MB
Max mailboxes per user	1500
Max mailbox nesting depth	128 levels
Default storage quota	1 GB per user
Max recipients per message	400
Max recipients per day	2000
Max forwards per day	2000
Max concurrent IMAP connections	15 per user
IMAP socket timeout	5 min 37 sec

These limits are configurable through the configuration files or the Settings API.