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Outbound SMTP

Outbound SMTP

WildDuck uses ZoneMTA with the zonemta-wildduck plugin as its outbound SMTP server. ZoneMTA handles message submission from users and delivery to external mail servers.

Role in the Mail Suite​

ZoneMTA serves as both the MSA (Mail Submission Agent) and the MTA (Mail Transfer Agent):

  • MSA (port 587): Accepts messages from authenticated users via SMTP submission
  • MTA: Queues and delivers messages to external mail servers

The zonemta-wildduck plugin integrates ZoneMTA with WildDuck's user database for authentication and policy enforcement.

Message Submission Pipeline​

When a user sends an email through ZoneMTA:

  1. Authentication — The user is authenticated against WildDuck's user database (supporting both main passwords and Application-Specific Passwords).
  2. From: address validation — The plugin checks that the From: header matches one of the user's registered email addresses or aliases. If not, the From: address is rewritten to the user's default address.
  3. Rate limiting — Outbound recipient limits are enforced (default: 2000 recipients per 24 hours, 400 per message).
  4. DKIM signing — Messages are signed with the appropriate DKIM key for the sender's domain.
  5. Sent Mail folder — A copy of the sent message is automatically uploaded to the user's Sent Mail folder in WildDuck.
  6. Queue — The message enters ZoneMTA's delivery queue (stored in MongoDB GridFS).
  7. Delivery — ZoneMTA delivers the message to the recipient's mail server, handling retries and bounce processing.

Local Delivery Bypass​

When a message is sent to a recipient on the same WildDuck server, the zonemta-wildduck plugin can bypass external SMTP delivery and deliver directly into WildDuck's database, avoiding unnecessary network round-trips.

ZoneMTA Features​

ZoneMTA provides additional capabilities beyond basic message relay:

  • Sending zones — Granular routing with virtual zones for different sender categories
  • IP warm-up — Gradual increase of sending volume for new IP addresses
  • Bounce handling — Automatic processing of delivery failure notifications
  • Queue management — Messages are queued in MongoDB GridFS for reliability
  • Prometheus metrics — Built-in monitoring metrics

Configuration​

The zonemta-wildduck plugin is configured via ZoneMTA's plugin configuration. Key settings include:

  • WildDuck API URL and access token
  • MongoDB and Redis connection strings
  • DKIM settings
  • Rate limit thresholds
  • Sent Mail folder behavior