How do repeated soft bounces become hard bounces?

Imagine you send to captain@deepcurrent.io and the delivery fails because the mailbox is full. That's a soft bounce. A temporary problem. No big deal. But what if it's still full next week, and the week after that?

At some point, your ESP stops treating it as temporary and starts treating it as permanent. That's when a soft bounce becomes a hard bounce, at least functionally.

Here's how it usually works. Your ESP keeps a bounce counter per address. Each failed attempt adds to that count. Once the address hits a threshold (typically 3 to 5 consecutive soft bounces, often within a 7 to 30 day window), the platform flags it as a permanent failure and suppresses it. From that point on, no more send attempts go out to that address.

The thresholds vary by platform. Mailchimp has its own rules. Klaviyo has different ones. Some ESPs let you configure the threshold yourself. Others handle it quietly in the background without surfacing much detail. It's worth checking your platform's documentation, or your bounce logs, to understand what rules you're actually operating under.

The logic behind this is sound. A mailbox that's been full for three weeks is almost certainly abandoned. A server that keeps timing out probably isn't coming back. Continuing to send wastes your sending reputation and drags down your engagement metrics. Suppressing it is the right call.

What this means practically is that not every hard bounce in your list started life as a hard bounce. Some of them were soft bounces that accumulated quietly over time. If you're doing list hygiene, it helps to know the difference (your ESP's bounce report will usually label these differently, if you dig in).

If you're seeing addresses drop off your list and you're not sure whether they were converted from soft bounces or something else, that's worth investigating before your next big send. Our SOS hotline is free if you want a second pair of eyes on your bounce data.