What is connection throttling?
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Connection throttling is when a receiving mail server intentionally slows down how many messages it'll accept from you at once. It's not rejecting your email. It's saying "okay, I'll take 10 connections at a time, but no more."
Why would a mailbox provider do this? Three big reasons. First, they want to protect their servers from getting overwhelmed. If you suddenly dump 100,000 messages at once, that's a resource load they need to manage. Second, throttling is a reputation signal. If they don't fully trust your IP or domain yet, they'll slow you down to see how you behave. Third, it's a spam defense mechanism. A sender who hits throttle limits and keeps trying to blast through anyway? That's a red flag.
What it looks like on your end: you'll see temporary deferrals in your logs. The receiving server returns a 4xx SMTP response code (like 421 or 450) that essentially says "try again later." Your sending MTA queues the messages and retries them according to your retry schedule. If you respect the throttle, your messages get delivered, just slower than you planned.
Common throttle thresholds vary by provider. Gmail is known to throttle new senders heavily until they build reputation. Yahoo Mail and Outlook have their own limits, though they don't publish exact numbers publicly. If you're sending through an ESP like SendGrid or Mailgun, they handle throttle management for you (they're already dealing with the providers' rate limits across all their senders).
And if you're hitting throttles repeatedly, that's usually a sign of one of three things: you're ramping volume too fast (especially on a new IP or domain), your sender reputation is shaky, or you're sending to too many addresses at one provider in one batch. The fix isn't to force more connections. The fix is to slow your send rate, warm your IP properly if it's new, and make sure your content and list quality are solid.
Throttling isn't the same as a hard bounce or a block. A throttle is temporary. A block means they're refusing your mail entirely. If you see throttles, your job is to back off and let the retry logic do its thing. If you see blocks, that's a bigger reputation problem.
Want to check if throttling is affecting your deliverability? Look at your MTA logs for 4xx codes and "too many connections" errors. If you're unsure what you're looking at, our SOS hotline can help you parse the logs and figure out whether it's throttling, blocks, or something else.
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