What is Hard Bounce vs Soft Bounce?

Definition

When an email cannot be delivered, the receiving mail server returns it as a bounce. Bounces fall into two distinct types, and the difference between them is the difference between permanent and temporary.

A hard bounce is a permanent delivery failure. The email cannot be delivered now and will not be deliverable later, at least not until something changes on the recipient's side. The most common cause is an address that simply does not exist, such as a mistyped address or an inbox that was closed when an employee left a company. A non-existent domain produces the same result. Because the failure is permanent, a hard-bounced address should be removed from your sending list right away.

A soft bounce is a temporary delivery failure. The address itself is real, but something blocked this specific message from arriving at this specific moment. The mailbox might be full, the recipient's server might be overloaded or briefly offline, the message might exceed a size limit, or the message might have been deferred by an anti-spam technique called greylisting. Because the failure is temporary, a soft-bounced address should not be deleted on the first failure. It should be retried, and only suppressed if it keeps failing.

How It Works

The hard or soft classification is rooted in the response codes of the Simple Mail Transfer Protocol, the system mail servers use to exchange messages. When your server attempts a delivery, the receiving server replies with a numeric status code, and the first digit of that code carries the meaning. A code beginning with 5 is a permanent failure, and a code beginning with 4 is a temporary failure.

Hard bounces are signaled by 5xx codes. A 550 typically means the mailbox is unavailable or does not exist, while 551, 552, and 553 cover related permanent problems such as a rejected recipient or a mailbox that is not local. These are definitive refusals, so the correct response is to suppress the address immediately and never retry it. Soft bounces are signaled by 4xx codes. A 421 means the service is unavailable, a 450 means the mailbox is temporarily unavailable, and a 452 often means insufficient storage. These are deferrals, so the correct response is to retry, typically up to three times across roughly seventy-two hours.

Many servers also return enhanced status codes, which add a three-part numeric structure such as 5.1.1. The first number repeats the permanent or temporary class, the second indicates the broad subject area such as addressing or mailbox status, and the third gives a specific detail. A well-built email platform reads all of these codes, classifies each bounce, retries soft bounces on a schedule, and suppresses hard bounces automatically. One nuance worth knowing is that a soft bounce which keeps failing for days, such as a mailbox that is permanently over quota or an account that has been abandoned, behaves like a hard bounce in practice and should eventually be suppressed too.

Why It Matters for Email Deliverability

Hard bounces are the more dangerous of the two for sender reputation. A high hard bounce rate tells mailbox providers that you are sending to addresses that do not exist, which is a classic signature of spammers and of senders working from stale or purchased lists. Providers respond by lowering your reputation, which means more of your legitimate mail is filtered into spam. Deliverability teams generally want the hard bounce rate to sit well below half a percent for a healthy campaign, and Gmail can issue permanent rejections once a sender's overall bounce rate climbs past two percent.

Soft bounces are less damaging individually, since the cause is often outside your control, but they still deserve attention. A rising volume of soft bounces, or the same addresses soft bouncing again and again, points to a list that is drifting out of date. The practical discipline is simple. Treat hard bounces as permanent and remove them at once. Treat soft bounces as temporary, retry them a few times, and suppress any that keep failing. Following that rule keeps your overall bounce rate inside the safe range and protects the inbox placement of every campaign that follows.

How VeriMails Handles It

The best way to manage hard bounces is to stop them before they happen, and that is the core job of VeriMails. Hard bounces are overwhelmingly caused by invalid and non-existent addresses, which is precisely what verification detects. When you check a list with VeriMails, each address is run through syntax validation, an MX and DNS lookup, and a live SMTP handshake that asks the recipient's server whether the mailbox actually exists. Addresses that would hard bounce are identified so you can remove them before the campaign is sent.

VeriMails also addresses the gray areas that drive unpredictable bounce behavior. Catch-all detection flags accept-all domains, where a missing mailbox can later produce a delayed soft bounce, and disposable address detection catches throwaway domains that are likely to be abandoned. VeriMails performs catch-all detection rather than scoring, so you receive a clear label instead of a guess. Verification runs through a REST API for real-time checks at signup or a bulk CSV upload for cleaning an existing list, with clear deliverability categories for campaign decisions. Verification starts at $0.0019 per email, with 10,000 credits for $19 and subscriptions from $15 per month, and every account begins with 100 free credits, no credit card required, that never expire.

Frequently Asked Questions