What is email throttling?
Email throttling happens when a receiving mail server intentionally slows down or temporarily rejects emails from a sender. Instead of accepting all your emails at once, the server accepts them at a controlled rate — or defers some with a “try again later” response. Throttling is different from blocking. A blocked sender is rejected entirely. A throttled sender is told to slow down and retry.Why providers throttle emails
Email providers throttle for several reasons:1. Volume protection
Mail servers have capacity limits. If a sender tries to deliver 100,000 emails in a minute, the server may throttle to protect itself and its users.2. Unknown sender
When a domain or IP has little or no sending history, providers are cautious. They accept a small batch first, observe engagement (opens, not-spam, etc.), and gradually accept more. This is one reason domain warmup exists.3. Reputation signals
If your recent emails have generated complaints or bounces, providers may throttle future emails while they reassess your reputation.4. Rate limits
Most providers have explicit rate limits per sender. When you exceed them, additional emails are deferred.How throttling looks
When a mail server throttles your emails, it responds with a 4xx temporary error code:4xx code means “temporary rejection” — the server is telling the sender to retry later. This is different from a 5xx code, which means permanent rejection.
Throttling by provider
Each email provider has different thresholds:Gmail
- Throttles based on domain reputation and sending volume
- New senders face stricter limits
- Reputation improves as engagement data accumulates
- Uses per-domain and per-IP limits
Outlook/Hotmail
- Has explicit rate limits that vary by sender reputation
- Can be aggressive with new or low-reputation senders
- Throttling often appears as
421temporary deferrals
Yahoo
- Throttles based on volume, reputation, and complaint rates
- Publishes a feedback loop to notify about complaints
How SendKit handles throttling
SendKit automatically handles throttled emails:- Automatic retries — when a receiving server returns a
4xxcode, SendKit queues the email for retry - Exponential backoff — retries are spaced out with increasing intervals to respect the provider’s limits
- Smart scheduling — for large sends, SendKit distributes delivery over time to avoid triggering throttles
Avoiding throttling
Warm up your domain
The most common cause of throttling for new SendKit users is sending too much too soon. Follow the warmup schedule and resist the urge to send high volumes early.Maintain your reputation
A good sender reputation means higher rate limits. Keep your bounce rate low, complaint rate minimal, and engagement high.Spread large sends over time
If you need to send a campaign to a large list, don’t send it all at once. SendKit distributes delivery automatically, but planning your sends helps too:- Segment your list and send to smaller groups over hours or days
- Send to your most engaged recipients first
Don’t send identical emails in bulk
Emails with identical content sent to many recipients at once are a spam pattern. Even for legitimate campaigns, personalizing content (recipient name, relevant details) helps avoid throttling.Throttling vs bouncing
It’s important to understand the difference:| Throttling | Bouncing | |
|---|---|---|
| Type | Temporary deferral | Permanent or temporary rejection |
| Server response | 4xx code | 5xx code (hard) or 4xx (soft) |
| Meaning | ”Slow down, try later" | "Can’t deliver this email” |
| Action | Automatic retry | Hard: suppress. Soft: retry |
| Impact on reputation | Minimal if handled properly | Negative, especially hard bounces |
FAQ
How long does throttling last?
How long does throttling last?
It depends on the provider and the cause. For new senders during warmup, throttling typically eases as reputation builds over days to weeks. For sudden volume spikes, it may resolve within hours once your sending rate normalizes.
Does throttling mean my emails won't be delivered?
Does throttling mean my emails won't be delivered?
Not necessarily. Throttled emails are deferred, not rejected. SendKit retries them automatically. Most throttled emails eventually get delivered, just with a delay.
Can I see which emails were throttled?
Can I see which emails were throttled?
Emails that are eventually delivered appear as delivered in your dashboard. Emails where all retries were exhausted appear as failed. The
email.delivery_delayed webhook fires when an email is being retried due to throttling.