Email Throttling: Why Large Sends Get Deferred
Email throttling means a receiving server accepts your mail more slowly than you send it. Simple as that. It temporarily refuses part of a campaign with a 4xx code, and your sending server queues those messages to try again later. So if a big campaign went out hours ago and a chunk of it, all headed to one mailbox provider, still sits there as deferred? That’s usually what’s going on. Below: what the receiving server is actually doing, how a deferral differs from a bounce, how retries work, what sets throttling off and how to spread your next send.
What happens when a mailbox provider throttles your email?
The receiving server hands some connections or recipients a temporary rejection and expects you to come back later. It doesn’t want to swallow everything at once. In SMTP (the protocol mail servers use to pass messages to each other), 4xx codes are temporary and 5xx codes are permanent. A 421 deferral basically says “not now, hold it and try again.”
Google’s list of Gmail SMTP errors includes responses like “This email has been rate limited” and “Email server has temporarily rejected this message”. And throttling is usually applied per provider. Which is why gmail.com can crawl along while every other domain delivers just fine.
Deferral vs bounce: why a deferred email is not lost yet
A deferral is a pause, then a retry. A bounce is a final answer: this message isn’t getting delivered. Here’s how the three outcomes stack up:
- Deferral: a 4xx response. Temporary, retried automatically, and the message can still land.
- Soft bounce: a temporary problem that simply outlasted the retry window.
- Hard bounce: a 5xx response. Permanent. That address should come off your list.
A reputation block is a different animal again, and the difference between bounces versus reputation blocks changes what you actually need to fix. Your hard and soft bounce reports show the final outcome once the retries are done.
How soft bounce retry works after a 421 deferral
Your sending server keeps the message in its queue and retries at growing intervals until it’s accepted or the retry window runs out. Why can mail sit deferred for hours? Because each retry can hit the very same limit if more volume keeps piling onto that provider.
While that’s happening, don’t resend the campaign by hand. And don’t push more volume at the same provider. Just watch whether the deferrals turn into deliveries or into soft bounces. MailCraft processes bounces in real time, retries soft bounces and removes hard bounces from your list automatically, so the list gets cleaned without anyone babysitting a spreadsheet after every campaign.
Why large sends trigger email throttling
Providers slow down senders whose volume or behaviour looks off compared with their own history, plus senders who fail the basic requirements. The classic trigger? A volume spike. A list that normally gets small weekly sends suddenly gets blasted with the whole database.
List quality counts too. Old segments nobody has mailed in ages tend to be full of inactive or dead addresses, and mailing them is one of the habits that hurt inbox placement. Then there’s setup. On that same Google page, Gmail says a message can be rate limited “because SPF authentication didn’t pass”, “because you’re not using a TLS connection”, or because the From: header “isn’t aligned with either the authenticated SPF or DKIM organizational domain.” Pretty blunt, honestly.
How to spread a large send and reduce sending rate limits hits
Split the campaign so each provider sees a volume pattern close to what it already knows from you. In practice, I’d go roughly like this:
- Before the send, check SPF, DKIM, DMARC alignment and TLS.
- Send to your most engaged subscribers first.
- Break the rest of the list into batches spread over hours or days.
- Hold back long-inactive segments, or give them their own re-engagement campaign.
- New list or new IP? Ramp volume up gradually.
- After each batch, check deferrals per domain and pause if they start climbing.
MailCraft plans include sending quotas that cap how much you can send in a given period. But a quota isn’t throttling. It has no say in how a mailbox provider paces your mail. Only the receiving side decides that.
Reading the signals after the campaign
Look at deferrals by receiving domain and over time, not just at the campaign-wide delivery rate (that one number hides a lot). Deferrals bunched up on one provider that clear within a few hours? Throttling. Deferrals that end as soft bounces, or turn into blocks, point to reputation or authentication trouble.
Keep the SMTP response text from your logs. The exact wording tells you whether it’s rate, SPF, TLS or alignment, and that’s what decides the fix.
Throttling is a temporary brake, not a failure. Fix authentication, respect engagement, spread the volume, and most deferred messages will get there on retry.
FAQ
Will deferred emails eventually be delivered?
Usually, yes. As long as your server keeps retrying and the limit lifts before the retry window closes. Anything still refused when the window shuts becomes a soft bounce. So check the final hard and soft bounce reports, not the live queue.
Should I resend a campaign stuck as deferred?
No. Resending just adds volume at the provider that’s already throttling you, which can drag the delay out even longer. And once the original retries go through, you’ve got duplicates in people’s inboxes.
Does a 421 deferral mean my domain is blacklisted?
Not on its own, since a 421 is a temporary response. That said, repeated rate limiting tied to failed SPF, missing TLS or misaligned From: headers points to a setup problem. Fix it before your next send.
