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Queues

Queues are essentially a holding area for outbound messages in an SMTP server. When a message arrives, it is placed in the queue until it can be delivered to its final destination. Stalwart SMTP supports an unlimited number of virtual queues, which means that a system administrator can create and configure multiple queues with different settings and behaviors. This allows for a high degree of flexibility and customization in managing incoming messages. For example, different queues can be created for different types of messages, such as messages from high-priority senders or messages with specific content, and these queues can be processed differently, such as by assigning more resources or prioritizing delivery.

Queues in Stalwart Mail Server are distributed and fault-tolerant, meaning that they can be distributed across multiple servers and are designed to continue operating even if one or more servers fail.

Storage​

The message queue can be stored in any of the supported data store backends while message contents are stored separately in any of the supported blob store backends. To specify the storage backend for the queue, use the storage.data and storage.blob attributes in the configuration file.

For example, to use the foundationdb store as the queue storage:

[storage]
data = "foundationdb"
blob = "foundationdb"

Schedule​

The queue scheduling settings determine the frequency of delivery attempts, the timing for notifications of delivery issues, and the maximum amount of time a message can stay in the queue before it is considered expired and returned to the sender. The following scheduling parameters can be configured under the queue.schedule key in the configuration file:

  • retry: A list of durations defining the schedule for retrying the delivery of a message.
  • notify: A list of durations specifying when to notify the sender of any delivery problems.
  • expire: The maximum duration that a message can remain in the queue before it expires and is returned to the sender.

Additionally, these scheduling settings can be made dynamic using expressions.

Example:

[queue.schedule]
retry = "[2m, 5m, 10m, 15m, 30m, 1h, 2h]"
notify = "[1d, 3d]"
expire = "5d"

Delivery Notifications​

Delivery Status Notifications (DSN) allows senders of an email message to be notified of its delivery status. It works by sending a message back to the sender, notifying them of the delivery status of the email message they sent. The delivery status information includes whether the message was delivered successfully, whether it was delivered to the recipient's mailbox or to a different location, whether it was delayed, or whether there was a permanent delivery failure.

DSNs are configured under the report.dsn key and support the following attributes:

  • from-name: The name that will be used in the From header of the DSN email.
  • from-address: The email address that will be used in the From header of the DSN email.
  • sign: The list of DKIM signatures to use when signing the DSN.

Example:

[report.dsn]
from-name = "'Mail Delivery Subsystem'"
from-address = "'[email protected]'"
sign = "['rsa']"