A soft bounce is the one people ignore. Unlike a hard bounce, the address is real, HubSpot keeps trying, and the message often gets through on a later attempt, so it never feels urgent. That's exactly why soft bounces are worth understanding. A single soft bounce is nothing. A contact that soft bounces on every send is a problem in slow motion, and left alone it can turn into a hard bounce and a dent in your deliverability.
This post covers what a soft bounce is in HubSpot, how HubSpot retries them, what actually causes them, and when a soft bounce is a harmless blip versus a warning sign. It sits under the broader topic of your HubSpot bounce rate.
A soft bounce is a temporary delivery failure. The address is valid and the mailbox exists, but something stopped delivery at that moment. Because the problem is temporary, the message may well deliver if it's tried again a little later, which is precisely how HubSpot handles it.
That's the key difference from a hard bounce, which is permanent and gets the address suppressed immediately. A soft bounce keeps the contact eligible for email, at least for now.
HubSpot doesn't give up on a soft bounce at the first failure. It retries the send over a limited window rather than dropping it straight away. If the message gets through during that window, it's recorded as delivered and you'd never know it briefly struggled. While it's still being retried, you may see the email sit in a pending status, which is essentially in limbo: it will usually resolve to either delivered or a soft bounce.
The part that matters is what happens when retrying doesn't work. HubSpot never resends on its own beyond that window, and if the same address keeps soft bouncing across multiple sends, HubSpot eventually treats it as a hard bounce and suppresses it for good. In other words, a persistent soft bounce doesn't stay a soft bounce forever. It graduates.
Soft bounces come from temporary conditions on the receiving side or with your own sending. The common causes:
Most of these resolve themselves. A full mailbox gets cleared, a busy server frees up, and the retry succeeds. The reputation-based ones don't resolve on their own, which is why the cause behind a soft bounce matters as much as the bounce itself.
Here's the practical rule: a one-off soft bounce is noise, but a pattern is a signal. If the same contact soft bounces repeatedly, or if your soft-bounce rate climbs across campaigns, that's usually not a coincidence of full mailboxes. It more often points to a deliverability problem, a sending IP or domain whose reputation is being questioned by receiving servers, which is a bigger issue than any single address.
So treat rising soft bounces the way you'd treat a rising temperature: not an emergency on its own, but a sign to look at what's underneath. Chronic soft bouncers are also on the path to becoming hard bounces, so catching the pattern early saves you both the suppression and the reputation hit later. If your soft bounces are climbing, it's worth stepping back to the wider picture of your HubSpot email deliverability.
Match the fix to the cause:
For size-related soft bounces, trim the email: lighter images, fewer heavy attachments, cleaner HTML. This is the one fully in your control.
For full-mailbox and temporary-server bounces, let HubSpot's retry do its job, and don't manually resend in a panic. If a specific contact soft bounces every single time, though, their mailbox may be permanently full or abandoned, which makes it a de facto dead address worth treating like a hard bounce.
For reputation-based soft bounces, the fix isn't the individual email, it's your list and your authentication. Sending to invalid and unengaged addresses is what erodes the reputation that triggers deferrals in the first place, so the durable fix is upstream: keep your list clean so receiving servers stop questioning your mail.
You can't stop a stranger's mailbox from filling up, but you can remove the soft bounces that are really list-quality and reputation problems in disguise. Keeping your list clean, so you're not repeatedly sending to abandoned mailboxes and so your sender reputation stays healthy, cuts the soft bounces you can actually control and stops the persistent ones from graduating to hard bounces.
That's where email validation earns its place. Validating addresses before you send catches the dead and abandoned mailboxes that would otherwise soft bounce their way to suppression, and doing it inside HubSpot means you catch them both in bulk and on entry, as new contacts arrive from forms, imports, and enrichment tools. Koalify runs this as a workflow action across your whole list and every new contact, with results written to a native HubSpot property so you can keep questionable addresses out of your sends. Teams running it have held sends to a 0.4% bounce rate across 4,000+ emails, hard and soft combined.
A hard bounce is a permanent failure (the address doesn't exist or the domain is dead), and HubSpot suppresses the address immediately. A soft bounce is a temporary failure (a full mailbox, a server that's down, a message that's too large), and HubSpot retries it over a limited window before giving up. Persistent soft bounces are eventually converted to hard bounces.
HubSpot retries a soft-bounced send over a limited window rather than dropping it at the first failure. If it delivers during that window it's counted as delivered; if it keeps failing, HubSpot stops trying. HubSpot doesn't resend on its own beyond that, and repeated soft bounces from the same address eventually convert to a hard bounce.
A single soft bounce, no. A pattern, yes. One contact soft bouncing once is usually a temporary issue on their end. The same contact soft bouncing every send, or a soft-bounce rate that climbs across campaigns, more often points to a deliverability or reputation problem worth investigating.
Trim oversized emails, let HubSpot's retry handle genuinely temporary issues, and treat chronic soft bouncers as dead addresses. For reputation-driven soft bounces, fix the cause upstream by keeping your list clean and your sending domain authenticated, so receiving servers stop deferring your mail.
Soft bounces are easy to ignore until the pattern behind them turns into hard bounces and a slipping reputation. See how many questionable addresses are already in your HubSpot, and clear them before they escalate.