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.

Anti-triggers

• Releasing or deleting a held message — this skill reports queue

contents and cannot change a message's disposition; use Mimecast Message Tracking.

• Finding one specific email — the queue shows current in-transit

state only. Anything already delivered, bounced, or rejected has left it; search with Mimecast Message Tracking.

• Why a message was blocked — queueing is a delivery outcome, not a

security verdict. For the threat reasoning use Mimecast Threat Intelligence.

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 Status

mimecast_get_queue_status

This tool takes no arguments. Its input schema is empty — there is no direction filter, no status filter, and no paging. Every call returns the same whole-gateway snapshot, and any narrowing happens in your own code after the fact.

Example response:

{
  "inbound": {
    "count": 12,
    "oldest": "2026-03-02T09:00:00Z",
    "details": [
      {
        "id": "eNqrVkpJLU...",
        "created": "2026-03-02T09:00:00Z",
        "status": "queued",
        "from": "[email protected]",
        "to": ["[email protected]"],
        "subject": "Purchase Order #4892",
        "reason": ""
      }
    ]
  },
  "outbound": {
    "count": 3,
    "oldest": "2026-03-02T08:30:00Z",
    "details": [
      {
        "id": "eNqrVkpABC...",
        "created": "2026-03-02T08:30:00Z",
        "status": "deferred",
        "from": "[email protected]",
        "to": ["[email protected]"],
        "subject": "Report Q1 2026",
        "reason": "550 5.1.1 The email account does not exist"
      }
    ]
  }
}

Key fields:

• inbound / outbound — the two queue directions, each an object. Either

may be absent; treat a missing key as "no data returned", not as zero.

• count — messages currently in that direction's queue
• oldest — an ISO 8601 timestamp, not an age in seconds. Compute the

age yourself as now − oldest before comparing against a threshold.

• details[] — per-message entries. **Every field is optional and the

array itself may be absent even when count is non-zero.** An agent that iterates details without checking count will report a clean queue on a backlogged gateway.

• reason — free text explaining the current state. It often carries the

SMTP response, but it is not guaranteed to, and it is not a parsed error code.

What this tool does not give you

The queue snapshot has no per-message retry accounting: there is no retryCount and no nextRetry. You cannot tell how many delivery attempts a message has had or when the next one is due, so any rule of the form "escalate after N retries" is not implementable against this tool. Use age (created) and reason instead.

To work with a subset of messages — deferred only, held only, one sender, one recipient — use mimecast_find_message, which does take filters, including status with queued, deferred, held, bounced, failed, delivered, accepted, blocked, and processing. The queue tool is a gauge; message tracking is the query interface.

Common Workflows

Daily Queue Health Check

1. Call mimecast_get_queue_status — it takes no arguments and returns

both directions at once

1. For each of inbound and outbound, derive the age of the backlog from

oldest (now − oldest), then compare:

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

1. Read count per direction. A rising outbound count with an ageing

oldest is the backlog signal

1. Scan details[].status for deferred entries and read their reason.

If count is non-zero but details is empty or missing, say so — do not report the queue as clean

Investigate a Stuck Message

1. Call mimecast_find_message with status: "deferred" — the queue tool

cannot filter, and message tracking is the only way to pull just the stuck messages

1. Rank candidates by age from created. There is no retryCount

available, so attempt-count heuristics do not apply

1. Read reason for the failure text. When it carries an SMTP response:

• 5xx — permanent rejection by the recipient server (invalid address, policy block)
• 4xx — temporary failure (server down, greylisting)

Treat reason as a hint, not a parsed code; it may be empty or prose

1. For 5xx, notify the sender that delivery failed permanently
2. For 4xx, confirm the recipient server is online; messages auto-retry
3. Use mimecast_get_message_info with the message id for full routing

and rejection detail

Detect a Downstream Outage

1. Call mimecast_find_message with status: "deferred" and, where you

already suspect a partner, recipient_address for that domain

1. Group the results by recipient domain yourself — neither tool aggregates

by domain

1. Many deferred messages to one destination domain with consistent 4xx

text in reason points at that recipient's server being down

1. Confirm the scale against mimecast_get_queue_status — the outbound

count tells you how much mail is affected in total

1. Notify the client that outbound delivery to that domain is affected and

messages will auto-retry

Identify Incorrectly Held Messages

1. Call mimecast_find_message with status: "held". Held mail is not a

queue-status filter — the queue snapshot has no held segment

1. Review held messages for false positives — legitimate emails held by

overly strict policy

1. For legitimate emails, use mimecast_release_message (see

message-tracking skill) to release them. Releasing is the plugin's one destructive tool: it delivers mail the platform decided not to deliver, and it cannot be undone

1. 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
• A 5xx response in reason means permanent delivery failure — these messages will eventually bounce; notify senders promptly
• 4xx deferred messages auto-retry — only escalate once now − oldest exceeds 2 hours
• Treat count as the source of truth for how much mail is queued, and details[] as a sample of it — never infer "queue is empty" from an absent details array
• Reach for mimecast_find_message whenever the question is about a subset; mimecast_get_queue_status only answers "how bad is it overall"

Related Skills

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