A queue growing during a campaign can have several causes. The application may be producing mail faster than its provider accepts it, the provider may be slowing outbound delivery, or a receiving system may be deferring particular destinations. Calling all three “Gmail throttling” hides the location of the bottleneck.
Locate the first delayed stage
References: RFC 5321 §4.2.1, RFC 5321 §4.5.4.1.
Capture timestamps for job creation, provider submission, provider acceptance and each delivery attempt. Preserve the full response text and the system that emitted it. A successful application API response is not the same event as the final receiving server accepting a message. If the provider does not expose a stage, mark it unavailable instead of inferring an exact timestamp.
SMTP distinguishes temporary negative replies from permanent negative replies. A temporary response is not a direction to retry continuously at full speed; the sending system needs a queue and an appropriate retry policy. A permanent failure should not be treated as an endless temporary backlog. RFC 5321 sections 4 and 4.5.4.
Inspect the changed workload
Compare destination mix, unique messages and attempts. A campaign sending many retries can inflate attempt counts without representing additional recipients. Separate marketing jobs from latency-sensitive transactional messages, and check whether they compete for the same concurrency or rate budget.
Google advises avoiding bursts and increasing traffic gradually while monitoring responses. Apply current Gmail guidance to its stated scope rather than translating it into a universal safe messages-per-second limit. No public number in this article can certify a sender's capacity. Gmail sender guidelines.
Work the queue arithmetic
References: RFC 5321 §4.2.1.
The local synthetic model starts with an empty queue. New jobs arrive at 120 messages per minute while the assumed service rate is 80 per minute for ten minutes. The backlog grows by 40 per minute to 400 messages. If arrivals are then paused and the same service rate continues, the idealized drain time is five minutes.
This is queue arithmetic, not a measured provider limit or a prediction of live recovery. It deliberately excludes retries, expiry and changing service rates. Those omissions explain why a real queue may take longer. Preserve the assumptions alongside the calculation so a capacity worksheet does not become an unsupported service-level promise.
The retained deterministic response-state model, rerun offline for this review, retains an attempt ledger. At 09:00 UTC, one scripted job receives “451 4.7.0 Temporary rate limit; retry later.” Its state becomes retry pending, the affected stream is marked for reduced pressure, and a controlled local retry at 09:30 receives “250 2.0.0 Accepted.” Another job receives “550 5.1.1 No such recipient” at 09:00 and enters permanent failure with no retry; its next action is approved recipient verification. These complete replies are invented local test inputs. Thirty minutes is this fixture’s retry setting, not a universal provider instruction. The code derives state transitions from response classes and forbids an unplanned second attempt.
Reduce pressure without losing correctness
References: RFC 5321 §4.5.4.1.
Pause or reduce the affected nonessential stream through the approved scheduler. Let the responsible queue retry temporary failures using its supported policy. Avoid creating a second queue that resends the same transaction because a dashboard is delayed. Use stable transaction identifiers to reconcile uncertain submission outcomes.
For urgent messages, follow the approved continuity route only after verifying authentication and duplicate prevention. Do not rotate infrastructure to evade a provider restriction. Escalate with timestamps, response text, destination scope, route identifiers and evidence of the corrective change.
Resume expansion when the backlog and age distribution stabilize under a defined legitimate workload. Continue checking the oldest messages: a low average delay can conceal a small group of severely overdue resets or receipts.