…and What to Do About Each
If you run cold outreach as a recruiter, you’ve bounced emails. The question isn’t whether it happens, it’s whether you understand what type of bounce you’re seeing, why it matters, and what to do next.
TL;DR
- An email bounce means your message was rejected by the recipient’s server before it reached the inbox.
- There are three types: hard bounces (permanent, remove the address), soft bounces (temporary, retry), and block bounces (your sending reputation is the problem, not the address).
- Recruitment contact data decays faster than almost any other B2B dataset. People change jobs, and that kills deliverability.
- High bounce rates damage your domain reputation, which affects every email you send, including Bullhorn notifications and interview confirmations.
- Verify your lists with ZeroBounce before sending. Not after. Before.
Most guides on this topic are written for generic email marketers who are sending newsletters to opted-in subscribers. That’s not what you’re doing. You’re sending cold outreach to candidates and clients from lists that come from LinkedIn, your ATS, CV databases, referrals, and data you’ve bought or scraped over the years. That data behaves differently. It decays differently. And bounces from that kind of list carry different consequences.
This guide is written for recruitment agencies. We’ll cover the three types of bounces (yes, three, we know most guides only mention two), how to read a bounce report when it lands in your Outlook or outreach tool, what the numbers mean for your domain reputation, and what you should actually do to reduce bounces before they start affecting emails you actually care about.
What Is an Email Bounce?
When you send an email, your sending server connects to the recipient’s server and essentially asks: “Will you accept this message?” If the answer is no, the message is returned to you with a reason code. That’s a bounce.
The reason code (sometimes called an SMTP code or bounce code) tells you whether the rejection is permanent or temporary, and whether the problem is with the address, the receiving server, or your own sending reputation.
That distinction matters enormously. Getting it wrong means either hammering dead addresses that damage your sender score, or permanently removing contacts you could have reached if you’d waited 24 hours.
The Three Types of Email Bounce
Hard Bounces
A hard bounce means permanent delivery failure. The message cannot, and will never, be delivered to that address.
The most common causes in a recruitment context:
- The email address no longer exists. The contact left the company. Their address was deleted. This is the single biggest source of hard bounces in recruitment. Your candidate and client data turns over constantly because people changing jobs is your entire industry.
- The domain no longer exists. The company has closed, rebranded, or moved to a different domain.
- The address was never valid. A typo, a fake submission, or a data entry error when the contact was added.
When you see a hard bounce, remove that address immediately and permanently. Do not retry it. Every subsequent send to a hard-bounced address chips away at your sender reputation with no upside.
In your bounce report, hard bounces typically show as a 5xx code. 550, 553, 554 are common. The message will say something like: 550 5.1.1 The email account you tried to reach does not exist.
Soft Bounces
A soft bounce means temporary delivery failure. The address is valid; the message just couldn’t be delivered right now.
Common causes:
- The recipient’s inbox is full. Less common with corporate accounts now, but still occurs.
- The recipient’s mail server was temporarily unavailable. Maintenance, overload, or a brief outage.
- Your message was too large. Heavy attachments or poorly formatted HTML emails can exceed server limits.
Your sending tool will typically retry a soft-bounced message automatically, usually several times over 24–72 hours. If it keeps failing, most tools will eventually reclassify it as a hard bounce and stop retrying.
Soft bounces appear as 4xx codes. 421, 450, 451 are common. The message will usually say something like: 421 4.7.0 Try again later.
The key thing with soft bounces: don’t panic, but do watch for patterns. If you’re seeing consistent soft bounces from a particular domain, the receiving server may be throttling you, which can be a warning sign that your reputation with that mail host is weakening.
Block Bounces
This is the third type that most generic guides fold into “hard bounce”, and they shouldn’t, because the cause and the fix are completely different.
A block bounce means the receiving server has rejected your message because of something about you as a sender, not because the address is invalid.
Common causes:
- Your sending domain or IP is on a blocklist. Spamhaus, Barracuda, and Microsoft’s own blocklists are the most impactful for recruiters doing outreach.
- You’ve been flagged as a spam sender by the recipient’s mail host. High spam complaint rates from previous campaigns can lead to domain-level blocks.
- Your authentication is broken or absent. If SPF, DKIM, or DMARC aren’t correctly configured, some receiving servers will reject your messages outright.
The difference from a hard bounce: the address itself is fine. If you fixed your reputation problem, you could reach that person. The block is about your domain, not their inbox.
Block bounces typically show as 5xx codes too, but the message will be different: 553 5.7.1 [BL23] Your message has been blocked. or 550 5.7.26 This mail is unauthenticated. If you’re seeing authentication references in your bounce messages, that’s a block bounce, and it requires you to fix your technical setup, not your list.
How to Read a Bounce Report
Whether you’re looking at a bounce report in Outlook, Woodpecker, Lemlist, or your ATS, the structure is broadly the same. Here’s what to look for.
The recipient address — obvious, but make a note. Multiple bounces from the same company domain can signal a block rather than individual bad addresses.
The bounce code — the 3-digit number (or extended code like 5.1.1) is your diagnostic tool. As a rule of thumb:
- Starts with 4 → temporary. Monitor and retry.
- Starts with 5 → permanent or blocked. Act on it.
The bounce message — read the actual text, not just the code. It will usually tell you why. “Account does not exist” is different from “Message rejected due to policy” is different from “Blocked by Spamhaus.” Each points to a different fix.
The pattern — this is where most people miss valuable information. Don’t just look at individual bounces. Ask:
- Are bounces clustered around one company domain? (Possible block.)
- Are bounces spread across many unrelated addresses? (List quality problem.)
- Did bounce rate spike after you sent a large campaign? (Possible reputation damage or sending too fast.)
- Are you seeing the same addresses bouncing repeatedly? (They should have been suppressed after the first bounce.)
In Outlook specifically: non-delivery reports (NDRs) appear in your inbox as a message from “Mail Delivery Subsystem” or similar. The bounce code and reason are usually buried in the middle of the message body. Look for the line beginning Diagnostic-Code: or Status: — that’s where the meaningful information is.
Why Recruitment Data Decays Faster Than Almost Any Other Dataset
Here’s a stat worth sitting with: ZeroBounce’s research found that email lists decay at roughly 22–25% per year on average across industries. In recruitment, that number is likely higher because the entire purpose of recruitment is facilitating job changes.
Every successful placement you make turns a valid contact into a potential bounce. Every candidate who leaves a company takes their email address with them. Every client contact who gets restructured out becomes a dead address in your ATS.
If you’re not actively verifying your contact data, you are absolutely, certainly accumulating hard bounces you don’t know about yet. They’re sitting in lists waiting to be sent to. And when you do send, they’ll fire off in a batch that your sending domain will take damage for.
The implication: list verification isn’t a one-time task. It’s a recurring operational step, particularly before any high-volume send. A tool like ZeroBounce will run your list against real-time checks (identifying invalid addresses, spam traps, role-based addresses, and catch-all domains) before a single email goes out.
What High Bounce Rates Actually Do to Your Domain
This is where recruiters sometimes underestimate the stakes.
When your bounce rate climbs (particularly your hard bounce rate) mailbox providers like Google and Microsoft take notice. Their systems interpret consistent bouncing as evidence that you’re sending to unverified, purchased, or low-quality lists. That’s associated with spam behaviour. So they begin treating your domain more sceptically: more emails get deferred, routed to spam, or blocked outright.
The knock-on effect matters especially for recruitment agencies, because your domain doesn’t just send cold outreach. It sends interview confirmations. Meeting requests. Terms of business. Placement paperwork. If your cold outreach damages your domain reputation, those transactional emails suffer too, including notifications from Bullhorn, Vincere, or whatever ATS you’re using to stay on top of placements.
Fixing this once it’s happened takes time. Preventing it by keeping bounce rates low is considerably easier.
What This Means for Recruiters
- Your contact data is inherently perishable. Build list verification into your workflow before any campaign, not as an afterthought when bounce rates climb.
- When you get a bounce report, read the code. A 4xx is not the same problem as a 5xx, and a block bounce is not the same as a dead address. Treating them all the same leads to either suppressing valid contacts or repeatedly damaging your sending reputation.
- A spike in bounces after a large send is a warning. Don’t send another large campaign until you understand what caused it.
- Your cold outreach domain reputation and your transactional email reputation are linked. Damage one and you may damage the other. Some agencies use a separate sending domain for cold outreach specifically to protect their main domain. A decision worth considering if you’re running high-volume campaigns.
- Below 2% hard bounce rate is the commonly cited safe threshold. In practice, the lower the better. Aim for under 0.5% if your list has been recently verified. In reality, you’ve probably seen this much higher, which means you’ve got some work tyo do in cleaning those lists.
FAQ
What’s the difference between a hard bounce and a soft bounce? A hard bounce is permanent. The address doesn’t exist or has been blocked, and retrying won’t help. A soft bounce is temporary. The address is valid but the message couldn’t be delivered right now. Your sending tool will usually retry soft bounces automatically.
What should I do when I get a hard bounce? Remove the address from your sending list immediately and add it to a suppression list. Do not retry it. Every subsequent send to a hard-bounced address damages your sender reputation further.
What does a block bounce mean? A block bounce means the receiving server has rejected your message because of something about your sending domain or IP, not because the address is invalid. It typically indicates a reputation problem: you may be on a blocklist, or your email authentication (SPF, DKIM, DMARC) may be misconfigured. The address may be perfectly reachable once the underlying issue is resolved.
How often should I clean my recruitment contact database? Before any high-volume send, as a minimum. Given the rate at which recruitment contacts change jobs, a list that was clean six months ago may have 10–15% invalid addresses by now. Running it through a verification tool like ZeroBounce before each campaign is a sensible standing practice.
My bounce rate is fine but emails still aren’t reaching candidates. Why? Bounces and deliverability are different things. An email can be “delivered” (not bounced) and still land in spam. If your bounce rate looks healthy but engagement is low, you may have a spam placement problem rather than a list quality problem which requires a different investigation. A deliverability audit will tell you which issue you’re dealing with.
Can I use the same sending domain for cold outreach and for ATS notifications? You can, but it carries risk. If your cold outreach campaigns generate high bounce rates or spam complaints, that damage applies to the whole domain, including your Bullhorn or Vincere notifications. Many agencies use a subdomain or separate sending domain for outreach specifically to insulate their main domain. It’s worth discussing with someone who understands how your specific setup is configured.
Ready to Get to the Bottom of Your Bounce Problem?
If your bounce rates are climbing, or you’re not sure what’s causing them, Quinset can run a full deliverability audit on your sending setup, covering list hygiene, domain reputation, authentication, and sending infrastructure. We work specifically with recruitment agencies, so we understand the data sources you’re working from and the tools you’re sending through.
