Mimecast Queue Management

Overview

The Mimecast delivery queue holds messages that are in transit — inbound messages being scanned and processed, outbound messages awaiting delivery to recipient servers. Queue monitoring is essential for detecting delivery backlogs, identifying stuck messages due to recipient server issues, and understanding the state of mail flow during incidents (e.g. a downstream mail server outage). A healthy queue processes messages within seconds; messages sitting in the queue for minutes or longer indicate a potential problem.

Key Concepts

Queue Types

Queue Message States

Retry Behavior

When Mimecast cannot deliver a message (e.g. the recipient mail server is down), it enters the message into a retry schedule:

• First retry: ~5 minutes
• Subsequent retries: exponential backoff, up to 4 days
• After 4 days without successful delivery: bounce notification sent to sender

API Patterns

Get Delivery Queue

mimecast_get_queue

Returns current queue contents with message counts and delivery status.

Parameters:

• direction — Queue direction: inbound or outbound (optional, returns both if omitted)
• status — Filter by status: queued, retrying, deferred, held (optional)

Example — Get full queue overview:

{}

Example response:

{
  "meta": { "status": 200 },
  "data": [
    {
      "direction": "inbound",
      "status": "queued",
      "count": 12,
      "oldestMessageAge": 45,
      "messages": [
        {
          "id": "eNqrVkpJLU...",
          "from": "[email protected]",
          "to": "[email protected]",
          "subject": "Purchase Order #4892",
          "received": "2026-03-02T09:00:00Z",
          "ageSeconds": 45,
          "status": "queued",
          "retryCount": 0
        }
      ]
    },
    {
      "direction": "outbound",
      "status": "deferred",
      "count": 3,
      "oldestMessageAge": 1800,
      "messages": [
        {
          "id": "eNqrVkpABC...",
          "from": "[email protected]",
          "to": "[email protected]",
          "subject": "Report Q1 2026",
          "received": "2026-03-02T08:30:00Z",
          "ageSeconds": 1800,
          "status": "deferred",
          "retryCount": 4,
          "lastError": "550 5.1.1 The email account does not exist",
          "nextRetry": "2026-03-02T09:45:00Z"
        }
      ]
    }
  ]
}

Key fields:

• oldestMessageAge — Age in seconds of the oldest message in this queue segment (high values indicate a backlog)
• retryCount — Number of delivery attempts made
• lastError — SMTP error from the last delivery attempt
• nextRetry — Scheduled time for next delivery attempt

Example — Outbound deferred queue only:

{
  "direction": "outbound",
  "status": "deferred"
}

Common Workflows

Daily Queue Health Check

1. Call mimecast_get_queue with no filters to get a full queue snapshot
2. Check oldestMessageAge for each queue segment:

• Under 60 seconds: healthy
• 60–300 seconds: minor delay, monitor
• Over 300 seconds: investigate

1. Check count for deferred messages — any deferred outbound messages need attention
2. For deferred messages, read lastError to diagnose the cause

Investigate a Stuck Message

1. Call mimecast_get_queue with status=deferred
2. Identify messages with high retryCount (5+) and long ageSeconds
3. Read lastError to diagnose:

• 5xx errors — Permanent rejection by recipient server (invalid address, policy block)
• 4xx errors — Temporary failure (server down, greylisting)

1. For 5xx errors, notify the sender that delivery failed permanently
2. For 4xx errors, confirm the recipient server is online; messages will auto-retry
3. Cross-reference with mimecast_find_message using the message ID for full delivery history

Detect a Downstream Outage

1. Call mimecast_get_queue filtering for direction=outbound and status=deferred
2. If many messages to the same destination domain are deferred with 4xx errors, the recipient server may be down
3. Check lastError for all affected messages — consistent errors to the same domain confirm an outage
4. Notify the client that outbound delivery to that domain is affected and messages will auto-retry
5. Monitor the queue; when the recipient server recovers, deferred messages will deliver automatically

Identify Incorrectly Held Messages

1. Call mimecast_get_queue with status=held
2. Review held messages for false positives — legitimate emails held by overly strict policy
3. For legitimate emails, use mimecast_release_message (see message-tracking skill) to release them
4. Document the release and consider adjusting the Mimecast policy to prevent recurrence

Error Handling

Queue Returns Empty When Delays Are Reported

Cause: The queue may have cleared by the time you query, or the affected messages may be in a different queue segment. Solution: Use mimecast_find_message with the specific sender/recipient to trace the message directly by its delivery status.

Persistent 5xx Deferred Messages

Cause: The recipient mail server is permanently rejecting delivery. Common causes: invalid recipient address, the recipient domain's MX records are wrong, or their server has a policy block against your client's domain. Solution: Notify the sender of the bounce reason (lastError content). If the recipient address is valid, ask the client to contact the recipient to have your domain allowlisted.

High Inbound Queue Count

Cause: Mimecast is processing a high volume of inbound messages, or scanning is taking longer than usual (e.g. an attachment sandbox backlog). Solution: Monitor the oldestMessageAge — if it grows over 5 minutes, contact Mimecast support. Temporary spikes during high-volume periods (e.g. start of business day) are normal.

Best Practices

• Run a queue health check at the start of each business day to catch overnight delivery failures
• A sudden spike in deferred outbound messages often indicates a recipient server outage — investigate by domain
• High retryCount with 5xx errors means permanent delivery failure — these messages will eventually bounce; notify senders promptly
• 4xx deferred messages auto-retry — only escalate if oldestMessageAge exceeds 2 hours
• Correlate queue data with mimecast_find_message for full message history when investigating specific reports

Related Skills

• api-patterns - Authentication, pagination, error codes
• message-tracking - Trace individual messages and manage held mail
• threat-intelligence - TTP threat logs