email bounce hard vs soft bounce explained

Last month I watched an agent send 200 emails to a list that hadn't been cleaned in two years. Within minutes, 38 of them bounced. The agent's sending domain got flagged, deliverability tanked, and every subsequent email from that address started landing in spam. Not because the content was bad. Because the agent didn't know the difference between a permanent rejection and a temporary one, and it kept retrying addresses that would never work.

That's the cost of ignoring bounces. And it's worse when an autonomous agent is doing the sending, because agents don't get tired and stop. They keep going.

Set up LobsterMail and your agent provisions its own address in under a minute.

What is an email bounce?#

An email bounce happens when a message can't be delivered to the recipient's mail server. The server sends back an error code explaining why. That error falls into one of two categories: hard bounce or soft bounce. The distinction matters because it determines what you do next.

The key takeaway: hard bounces mean "stop sending here forever." Soft bounces mean "try again later, but pay attention if it keeps happening." Mishandling either one will erode your sender reputation, which affects every email you send from that domain going forward.

Hard bounces: permanent, non-negotiable#

A hard bounce means the email cannot be delivered and never will be. The most common SMTP error code is 550, which translates to "requested action not taken: mailbox unavailable." You'll also see 550 5.1.1 (user unknown) and 550 5.1.2 (domain not found).

Common causes:

• The email address doesn't exist. Someone typed jon@gmial.com instead of john@gmail.com.
• The domain has no mail server. The DNS has no MX record, so there's nowhere to deliver the message.
• The receiving server has permanently blocked your sending domain or IP.

Here's the part most guides skip: not every 550 is truly permanent. Some strict security filters return a 550 for messages they don't trust, even when the address is valid. Greylisting systems can also produce what looks like a hard bounce on the first attempt, then accept the message on retry. Mailchimp briefly mentions this edge case, but most platforms treat all 550 responses as final.

If you're building agent-based email workflows, this gray area matters. A blanket "suppress all 550s" rule might remove valid addresses that simply have aggressive spam filters. The better approach is to check the full SMTP response text, not just the code. A 550 user not found is genuinely permanent. A 550 message rejected by policy might not be.

What to do about hard bounces#

Remove the address from your list immediately. Don't retry. Don't wait. Every attempt against a dead address counts against your sender reputation with mailbox providers like Google and Microsoft. If your hard bounce rate exceeds 2%, most ESPs will start throttling your sends. Google's bulk sender guidelines updated in 2024 set the threshold even lower for high-volume senders.

We covered the broader implications of this in our guide on email deliverability for AI agents, but the short version is: mailbox providers track your bounce rate as a signal of list quality. A high rate tells them you're either scraping addresses or not maintaining your list. Either way, you look like a spammer.

Soft bounces: temporary, but not harmless#

A soft bounce means the email couldn't be delivered right now, but the address itself is valid. The server is saying "come back later." Common SMTP codes include 421 (service not available, try again), 452 (insufficient storage), and 422 (mailbox full).

Common causes:

• The recipient's mailbox is full. This is surprisingly common with free email providers.
• The receiving server is temporarily down or overloaded.
• Your message exceeds the server's size limit.
• Rate limiting: you've sent too many messages to that domain in a short window.
• DNS resolution failed temporarily.

Most email providers retry soft bounces automatically. Mailchimp retries for up to 72 hours. HubSpot retries several times over a shorter window. The retry logic varies, but the principle is the same: give the server time to recover, then try again.

When soft bounces become a problem#

A single soft bounce is noise. Ten soft bounces to the same address over three campaigns is a pattern. If an address soft bounces consistently, most ESPs will eventually reclassify it as a hard bounce and suppress it. The threshold varies: some platforms suppress after 3 consecutive soft bounces, others wait for 7.

The risk for agents is different than for human-managed campaigns. An agent running an automated outreach workflow might hit the same full mailbox every day for a week, accumulating soft bounces that eventually get reclassified. Without monitoring, the agent doesn't know its effective bounce rate is climbing. We wrote about how to structure agent sending to avoid this kind of spiral in our piece on building an AI sales outreach agent that doesn't get blacklisted.

Can a soft bounce become a hard bounce?#

Yes. If a mailbox stays full for weeks, the provider may eventually deactivate the account. What started as a 452 insufficient storage becomes a 550 user not found. More commonly, your ESP will reclassify it after repeated failures. The address itself may still exist, but from your sending infrastructure's perspective, it's dead.

SMTP bounce codes worth knowing#

Most articles about bounces stop at "hard = permanent, soft = temporary." But if you're working with email at the infrastructure level, you'll encounter raw SMTP codes. Here are the ones that matter:

Hard bounce codes:

• 550 5.1.1 — User unknown. The address doesn't exist.
• 550 5.1.2 — Domain not found. No MX record in DNS.
• 550 5.7.1 , Delivery not authorized. You're blocked by policy.
• 551 5.1.6 , Address has moved, no forwarding.

Soft bounce codes:

• 421 4.7.0 , Connection rate limited. Slow down.
• 450 4.2.1 , Mailbox temporarily disabled.
• 452 4.2.2 , Mailbox full.
• 451 4.3.0 , Mail server temporarily unavailable.

Gray area codes:

• 550 5.7.1 can sometimes indicate a temporary policy block (greylisting) rather than a permanent rejection. Check the response text.
• 451 4.7.1 , Greylisting. The server wants you to retry after a delay. This looks like an error but is actually an anti-spam mechanism. Retrying after 5 to 15 minutes usually succeeds.

If you're using LobsterMail's infrastructure, bounce classification happens automatically. The SDK parses the SMTP response, categorizes the bounce type, and either retries (soft) or suppresses (hard) without the agent needing to interpret error codes manually.

What's an acceptable bounce rate in 2026?#

The benchmarks have tightened. After Google and Yahoo rolled out stricter bulk sender requirements in early 2024, the acceptable hard bounce rate for most mailbox providers is now under 2%. Some deliverability consultants recommend staying under 0.5% for cold outreach.

Here's how to think about it:

• Under 0.5%: You're fine. Your list is clean.
• 0.5% to 2%: Watch it. Clean your list and verify addresses before sending.
• Over 2%: You're likely being throttled already. Stop sending, clean the list, and check whether your domain or IP has been flagged.

Soft bounce rates are harder to benchmark because they're transient. But if more than 5% of your sends are soft bouncing, something systemic is wrong: either your sending volume is too high for your reputation, or you're hitting a lot of inactive mailboxes.

How to prevent bounces before they happen#

The best bounce handling is the kind you never need. A few practices that actually work:

Verify addresses before sending. Email verification APIs can catch typos, nonexistent domains, and deactivated accounts before you burn a send on them. This is the single most effective thing you can do.

Don't send to old lists. If someone gave you their email 18 months ago and hasn't opened anything since, verify the address before your next campaign. Addresses go stale faster than people expect.

Warm up new sending domains. A brand-new domain sending 5,000 emails on day one will trigger every spam filter on the internet. Start with 50 sends per day and ramp up over 2 to 4 weeks.

Monitor bounce rates per campaign, not just overall. A single bad list segment can spike your bounce rate and drag down deliverability for everything else you send.

Bounces vs. spam complaints: different problems, different solutions#

A bounce means the email wasn't delivered. A spam complaint means it was delivered and the recipient marked it as junk. Both hurt your sender reputation, but they signal different issues. High bounces say "this sender has a bad list." High spam complaints say "this sender is annoying people."

Google now requires bulk senders to maintain a spam complaint rate below 0.3%. That's 3 complaints per 1,000 delivered emails. Exceed it consistently and you'll end up in the spam folder for everyone, not just the people who complained.

For agents, this distinction matters because an agent might optimize for low bounces (by cleaning its list) while ignoring complaint rates (by sending too frequently or to uninterested recipients). Both numbers need monitoring.

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