INBOX PLACEMENTEmails bounce for seven concrete reasons: spam filters, dead addresses, blacklisted IPs, downed servers, full inboxes, low sender reputation, and admin blocks. Hard bounces (dead address, blacklist) destroy sender reputation. Soft bounces (full inbox, server down) are temporary. Fix the root cause by verifying lists, monitoring blacklists, and setting SPF and DKIM records before launching any cold email campaign.
I know this is terrible metaphor… but you get my point.
Bounces are a common occurrence that plague email campaigns and while it's impossible to eliminate them completely, there are a few surefire ways to drastically reduce them.
Soft bounce vs. hard bounce
A soft bounce indicates temporary reason why an email cannot be delivered -such as a full inbox or an unavailable server- whereas a hard bounce means that your email will not be delivered at all -because of an non-existing address or because of blacklisting.
Hard bounces are the ones effectively affecting your sender reputation because they're associated with spammy practices that include sending mass email to unverified purchased lists and/or including various spam triggers.
Check out the main reasons emails bounce and what you can do about it!
Attention! At the end of this post, I'll teach you about SPF and DKIM records and how setting them will substantially increase your deliverability!
1. SPAM filters
Type: Hard bounce [CNIL]
According to Statista's 2025 spam traffic data, spam still accounted for 45% to 49% of all email traffic globally, with phishing and malware payloads dominating the AI-generated wave.
That is why SPAM filters keep getting more aggressive in 2026. Gmail and Microsoft now require SPF, DKIM, DMARC enforcement (since February 2024), one-click unsubscribe headers per RFC 8058, and bulk-sender thresholds under 0.3% spam rate. Corporate servers have always been stricter and reject anything that looks templated or comes from a cold IP.
What you can do about it:
Your job is then to avoid tingling spam triggers. There are a few ways you can do that:
- Watch your subject line
- Avoid all caps, exclamation points and using red fonts
- Use a reliable ESP
- Use links carefully
- Keep your emails short
- Don't attach files
- Set up your DKIM & SPF records
Check out our in-depth article about how to keep out of the spam folder.
2. Email address doesn't exist
Type: Hard bounce [HBR]
As time goes by, I'm more and more amazed that people would work at the same company for more than 5 years. Legend has it that people used to work at the same company for their whole life!
This doesn't happen much anymore. According to Economic News Release from the the Bureau of Labor Statistics (US), the median tenure for workers age 25 to 34 is 3.2 years. When employees go, their email addresses tend to be deactivated, which leads to cold, hard bounce.
What you can do about it:
Keep your lists up to date and check the validity of the email addresses you plan to write to.
Overloop offers a verifying tool, but you don't need it to get started. If you so much as check your prospects on LinkedIn to make sure they still work at the same company, you're already ahead of the game.
3. IP is blacklisted
Type: Hard bounce
All emails sent by the users of an Email Service Provider go through the same IP address -the IP of the ESP. Which means that if one user messes with the IP of the ESP, all users might get in trouble and see their emails bounce.
What you can do about it:
Team up with a reliable ESP. The one we use and recommend is Google. Not only are they super reputable, they also detect and prevent unlawful practices such as mass emailing purchased lists of prospects.
4. Receiving server is down
Type: Soft bounce
It can happen that the server of the recipient is unavailable at the moment, which will be categorized as a soft bounce. It's generally safe to give it another try. Email marketing services like Sendgrid keep attempting to send the message for another 72 hours in case of a soft bounce.
What you can do about it:
This isn't something you can do much about. It's extremely rare for the email server of a company to be down for more than a few hours, so in case you haven't been able to get your email across after a day or two, it might be worth checking if the company… didn't just close up shop, which can happen overnight with startups, even with massive funding.
5. Inbox is full
Type: Soft bounce
In this day and age, a full inbox is not a current occurence. A full mailbox usually means and old and non maintained mailbox; it doesn't really make sense to pursue sending emails to this address.
What you can do about it:
Be wary of old ESP's that are rarely ever used but are still active like Netscape or AOL. I'm not shaming people who still use those -one of my best friend does- but these addresses pointing to those domains probably indicate a full or cancelled inbox. It's worth checking beforehand if the account is still in use.
6. Sender reputation is too low
Type: Hard bounce
Your sender reputation is your main asset when it comes to sending emails. It's associated to the email server you are using to send messages from. Every ISP (Internet Service Provider) has different algorithms when it comes to determine an IP's reputation. If the ISP suspects your IP of SPAM, they will block your email, which will make your emails bounce. Which is a problem since hard bounces are the most damaging to your reputation.
Another reason why your reputation score might be low is that your IP is brand new and is thus unknown to other servers. And, like humans, servers are suspicious of what they don't know.
What you can do about it:
Clean your lists. If more than a 10% of your emails bounce, you're up for huge red flags. The reason for this is that spammers mass email large purchased lists full of invalid email addresses. If your emails bounce a lot -or you experience bounce spikes- ISP's might think you're doing just that. And if you actually are, you need to stop. Now.
In case your sending domain is brand new, you should consider Active Campaign's approach; they compares sender reputation to credit history. You may not have done anything bad but since you haven't done anything at all, how should anyone know if you're trustworthy?
7. Server administrators
Type: Hard bounce
Beyond ESP's, servers admins can themselves flag emails as SPAM or implement tools like SpamAssassin that will grade every incoming email and give it a score. That score depends on an exhaustive list of criteria that varies from one tool to another.
What you can do about it:
They may not all do it but SpamAssassin does inform you when they bounce your email back. They have published a list of tips in order to avoid false positives and you can contact them if you think you're being blocked because of a bug. If none of that works and you genuinely believe your emails to belong in your recipient's inbox, you can then try and contact the server admin personally and ask for your domain to be whitelisted.
Technical but invaluable: SPF and DKIM records
This may not be advertised and done enough because of its technical aspect, but setting your DNS records is an important step in making your overall deliverability better.
If you use sales automation or email marketing services to deliver emails on your behalf, pay close attention to what follows.
SPF (Sender Policy Framework)
To make it simple, your SPF defines which IP addresses can be used to send emails from your domain.
Servers are like messengers, passing on mail from you to your recipient. By setting your SPF -which can only be done from your own domain- your sending server can prove to your recipient's receiving server that you gave it permission to transmit an email on your behalf. If you don't, it could make emails bounce back.
We, here at Overloop -as well as most of our clients- deliver emails through Google's servers; so here are their instructions on how to set your SPF.
It's unfortunately impossible to make a step-by-step guide for DNS settings since the process depends on every domain host. Here are, nonetheless the link to the instructions for some of the most popular ones:
- Amazon Web Services: Configuring DNS, Resource Record Types
- Dreamhost: SPF, DKIM
- GoDaddy: Add a CNAME Record
- Google Domains: DNS Basics
- Hostgator: Manage DNS records
- Hover: Edit DNS Record
- Namecheap: SPF & DKIM
- Squarespace: Advanced DNS Settings
- Stablehost: How do I get to cpanel?
Want emails that land in the inbox, not spam?
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Try free →Book a demoDKIM (DomainKeys Identified Mail)
Much like SPF, DKIM is a way to identify you as the real sender of the email. It works as some kind of seal.
You're actually setting up two keys:
- A private one, which is encrypted, unique to your domain, only available to you and which you use to encrypt your signature in the header of your emails
- A public one, which you add to your DNS records using DKIM standard to allow your recipient’s server retrieve it and decrypt your hidden signature from the header of your email
Again, if you're using Google, here's their guide. For the rest, it depends on your domain host.
In a word…
It's simply a matter of following best practices: keep clean lists, operate from a reputable ESP, avoid to tingle spam filters and set your DNS records.
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