When a high-converting email flow suddenly stops sending, e-commerce businesses feel it in their bottom line. That’s precisely what happened when an online retailer’s abandoned cart emails came to a grinding halt. The culprit? SendGrid’s rate limits. What followed was a scramble to restore crucial email sequences, salvage lost revenue, and build a smarter, throttle-based email strategy that could stand up to technical constraints.
TL;DR:
One company’s abandoned cart email series stopped after hitting SendGrid’s rate limits. As conversions plummeted, they traced the problem to API request bottlenecks caused by sudden traffic spikes. The solution? A smart throttle strategy that prioritized sends and restructured batch delivery. This restored email flow and protected future sales during peak hours.
The Disruption Nobody Expected
The online storefront had invested heavily in its marketing automation platform. Conversion-optimized sequences were in place—including one particularly effective abandoned cart series, which routinely brought in 10–15% more revenue each month.
The team had scaled up its promotional campaigns, unknowingly creating a hidden bottleneck. As campaign traffic surged, many customers who added items to their cart and left without purchasing were never re-engaged.
At first, there was no immediate red flag. Conversions slowly dipped, and it wasn’t until a technical audit that they discovered the emails weren’t going out at all. The culprit? SendGrid’s rate limits had been silently rejecting a portion of their transactional emails.
Understanding the SendGrid Rate Limit
SendGrid, like many transactional email services, implements rate limits—restrictions on how many emails can be sent through their API in a given timeframe. These controls are in place to prevent spam, improve deliverability, and manage infrastructure load.
There are two key aspects to understand:
- API request limits – The number of requests your system can make per second/minute.
- Email throughput limits – The number of actual emails per user or per account that can be delivered over time.
When a system surpasses those limits, SendGrid begins throttling—essentially holding or dropping requests until conditions normalize. Unfortunately, when the team’s campaign sent spikes of traffic through the same API endpoint as the abandoned cart flow, SendGrid deprioritized the latter. The result? Thousands of missed recovery opportunities.
The Silent Revenue Leak
The impact was significant. Within two weeks, the retailer noticed:
- A 22% drop in recovered checkouts
- Increased inventory holding times
- Customer service inquiries asking about “forgotten” discounts
The team realized that the damage wasn’t just limited to sales. Customer experience was being affected. Internal teams were wasting time chasing incomplete orders. If left unchecked, this small crack could turn into a long-term operational drag.
The Diagnosis: Traffic Spikes Leading to Failures
Working alongside their ESP support and engineering team, logs revealed that email sends were spiking dramatically whenever a flash promotion or influencer campaign launched. Background processes that triggered abandoned cart emails ran into delays, retries, and eventual drop-offs.
Important webhook notifications were being queued or discarded. The system simply couldn’t push the volume needed fast enough to match user behavior in real time.
The Throttle Strategy That Saved the Cart
Rather than throwing money at higher-tier API privileges or upgrading their email service (a temporary fix at best), the company adopted a smarter approach: throttle-based email dispatching.
The Core Components of the Strategy:
- Segmentation of Email Priorities: High-value transactional flows (like abandoned cart) were isolated from marketing campaigns, given priority queues.
- Rate Adaptive Throttling: An internal middleware monitored SendGrid’s API response latency and usage headers. During high-load periods, send rate was scaled back intelligently, avoiding hard limits.
- Event Batching: Instead of triggering one email per cart event, requests were batched into grouped time intervals (every 2–3 minutes), reducing API overhead.
- Timeout & Retry Policies: Failed email attempts used exponential backoff with maximum retry caps to prevent fruitless overloads.
The results were immediate. Within 48 hours of implementation:
- Abandoned cart email success rate jumped from 58% to 94%
- Conversion rates returned to seasonal benchmarks
- API error rate dropped by 83%
Looking Ahead: Resilience by Design
While email platform rate limits may seem like an invisible barrier, they often reveal deeper issues—dependency on a single pipeline, lack of prioritization logic, or brittle architecture. This company used the setback as a chance to build infrastructure that thinks ahead.
They also did the following:
- Set up separate API keys for transactional vs. bulk sends
- Implemented fallback SMTP sending for critical flows
- Introduced monitoring dashboards with alerting on email failure rates
Today, not only does the company’s abandoned cart email work flawlessly under load, it does so more intelligently than ever before.
Key Takeaways
- Rate limits are not just a platform constraint—they’re a signal to optimize delivery strategy.
- Critical email flows should always be prioritized and separated from marketing sends.
- Throttling isn’t about slowing down—it’s about controlling acceleration to reach completion efficiently.
- Middleware, retry policies, and batching ensure stability during high-volume events.
Frequently Asked Questions (FAQ)
Q1: What caused SendGrid to rate limit abandoned cart emails?
A: A surge in API requests due to overlapping marketing campaigns exceeded the limit, causing transactional emails to be delayed or dropped.
Q2: Can API limits be increased?
A: Yes, SendGrid offers higher rate tiers, but at additional cost. The better long-term approach is to control traffic intelligently through throttling and segmentation.
Q3: How does batching reduce email failures?
A: Batching groups multiple email events into fewer API requests, reducing the number of calls and improving stability during peak loads.
Q4: How quickly did the throttle strategy improve performance?
A: Improvements were noticeable within 48 hours, with cart success rates climbing back above 90% shortly after throttling and retries were implemented.
Q5: Can this approach be used with other ESPs?
A: Absolutely. The core strategy—prioritizing, monitoring, and throttling intelligently—applies to any email provider with rate limits or delivery caps.
