Can bounces trigger throttling?
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Your delivery is slowing down. Emails that normally land within minutes are taking hours. Sound familiar? There's a good chance your bounce rate triggered throttling at the receiving ISP.
Throttling is when an ISP deliberately slows down how many emails it accepts from you per hour (or per connection). It's not a block. It's more like a bouncer at the door saying "not so fast." When an ISP sees a flood of bounces coming from your sending domain or IP, it treats that as a signal that something's off with your list, and it pulls back the welcome mat.
Here's how the cycle usually plays out. You send a campaign. A chunk of addresses are invalid or dormant. The ISP returns a wave of "unknown user" bounce codes. The ISP's filters notice the ratio and start rate-limiting your connections. Your delivery logs start filling up with 4xx deferral codes instead of 2xx acceptance codes. Everything slows to a crawl.
The three scenarios that trigger this most reliably are a sudden spike in bounces (which suggests a stale or purchased list), a new IP sending to an old list (which combines bounce risk with the lack of a reputation to offset it), and gradual bounce accumulation that crosses an ISP's internal threshold over time.
What counts as "high"? A hard bounce rate above 2% will get most ESPs and ISPs paying attention. Above 5% and you're likely already triggering protective measures at major mailbox providers like Gmail and Outlook. Hard bounces (permanent failures like invalid addresses) are weighted much more heavily than soft bounces (temporary failures like a full mailbox). If you're unsure which type is driving your numbers, your sending logs will show the specific bounce codes.
To break the cycle, start by stopping the bleeding. Suppress all hard-bounced addresses immediately. Don't keep retrying them. Then reduce your sending volume while your metrics recover. ISPs reassess sender reputation over time, and lighter sends with better engagement give that score room to improve. If your list hasn't been cleaned in a while, that's worth doing before your next campaign goes out.
Throttling usually lifts on its own once your bounce rate drops and your delivery metrics look healthier. There's no formal appeal process at most ISPs. You earn your way back by sending better.
Still if your list feels like it needs a proper clean before you send again, we do that (hi ;). You can also check whether your domain has picked up any blocklist entries alongside the throttling, since the two often show up together.
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