Email & SMTP · 1 min read
Rate-Limited Emails: Resolve Throttling
What's happening
Receiving servers are limiting how fast they accept mail from your domain or IP. Emails are delayed or temporarily rejected with messages like:
- "The recipient's server is temporarily limiting incoming messages due to high volume"
- "Too many messages, try again later"
- "421 rate limited"
These are usually soft bounces, and many will be retried automatically.
Common causes
- Sending a large volume to one provider (such as Gmail) in a short time
- A new or recently changed sending domain that hasn't been warmed up
- Low engagement, which makes providers more cautious
Fix it
- Spread sends out. Use batch scheduling to send in smaller groups over hours or days.
- Warm up new domains before large campaigns.
- Send to engaged contacts first to build trust.
- Avoid sudden spikes. Keep daily volume consistent.
- Check authentication. Unauthenticated mail is throttled more aggressively.
When to worry
Occasional throttling is normal. If most of a campaign is rate limited, or the bounces become permanent, slow down and review your reputation in Google Postmaster Tools.