What should I do if my emails get throttled - Online Help | Zoho Campaigns

What should I do if my emails get throttled

In short: throttling is a warning, not a wall. The fix is to slow down to the providers that are deferring you, keep sending only to engaged contacts, and let your reputation recover — while making sure your domain is authenticated and your list is clean. Left unaddressed, throttling escalates to spam-foldering, blocks, and blacklisting.
If mailbox providers are skeptical of your email volume or sending practices, they may defer your emails. This limiting of, or delay in, accepting email is called email throttling.
In practice, throttling shows up as SMTP 4xx "soft" responses (for example, 421 4.7.0 Try again later or "rate limit exceeded"). A 4xx is a temporary "slow down," unlike a 5xx permanent rejection.

Email throttling – An infographic.


When you send emails to your recipients, any of the following can happen:
  • Delivered — the message reaches your recipient’s inbox.
  • Bounced — the message isn’t delivered and is returned with an error explaining why (a permanent 5xx failure).
  • Throttled — the recipient system won’t accept the message right now and holds it in a queue. Zoho Campaigns automatically retries on an escalating schedule, which delays delivery. If the recipient system keeps refusing the queued mail for too long, it will eventually bounce.

Your emails may get throttled if:

  • You’re new to email marketing or using a new business domain and send beyond a recipient system’s rate limit. With no sending history, your mail looks suspicious because there’s no record of how recipients responded to your domain before.
  • You’ve had negative responses — spam markings, unsubscribes, and abuse complaints — on previous emails.
  • Your previous emails have a high bounce rate.
  • You send to spam traps set by that recipient system.
  • Your email content and sending practices resemble spam.
  • You send too fast — a large burst all at once, or too many simultaneous connections to one provider — even if your reputation is otherwise fine.
Recipient-system restrictions do affect deliverability, but planning your sends around the best practices below keeps your email from being throttled — and can improve contact engagement as an added benefit.

How to avoid email throttling

Send to your most engaged contacts first

Start with the contacts you communicate with often, who are clearly interested and likely to respond positively. Target them first; once you’re confident your mail isn’t being throttled and deliverability is healthy, gradually add less-engaged contacts. Leading with engagement is one of the strongest positive signals you can send a recipient system.

Authenticate first, then warm up your domain

Before anything else, make sure SPF, DKIM, and DMARC are in place — since February 2024 they’re required for bulk senders, and unauthenticated mail is one of the strongest negative signals a provider can see. A new sender domain is "cold": recipient systems have no prior fingerprint of conversations between you and your recipients, so a sudden high volume from a cold domain gets throttled, blocked, or marked as spam.

Warm up gradually instead. A safe pattern is to start small (roughly 20–50 emails a day in week one), then increase by no more than about 25% per week as long as your metrics stay clean, pausing the ramp at the first sign of rising deferrals. Because providers throttle independently, warm up each one on its own schedule rather than a single blended calendar.

Slow down for the providers that defer you

Throttling is usually concentrated at specific recipient domains. When you see sustained 4xx deferrals from one provider, reduce your hourly send rate to that provider and hold volume steady for 48–72 hours before ramping again. A key rule: don’t flush the queue or resend manually — your system is already retrying, and forcing a bigger burst at a server that’s throttling you only deepens the block. If deferrals suddenly spike across all providers at once, check whether your domain has landed on a blocklist.

Spread volume and distribute a crowded contact base

Sending an entire campaign in one burst invites throttling. Spread the same volume across several hours instead of firing everything at 9 a.m. It also helps to split heavily concentrated domains across segments.

For example, suppose Patricia has most of her contacts at zylker.com, and zylker.com throttles her mail beyond 100 per day. She divides her 1,000 zylker.com addresses across 10 segments, each also mixing in addresses from other domains that don’t throttle her. She sends the segments on a staggered schedule so no single domain receives too much at once. Identify the recipient domains that throttle you and segment their addresses the same way.

Read your bounce and error messages

Your campaign report is the fastest diagnostic. Look for 4xx messages such as Too many emails in short duration, "rate limit exceeded," or "too many connections," and group them by recipient domain to see who is throttling you. When you find them, reschedule and slow your sends to those domains. Monitor your sender reputation over time in Google Postmaster Tools and keep your spam-complaint rate below 0.1% so throttling doesn’t escalate.