Almost everyone starting with cold email thinks the problem is in the copy. It rarely is. If your open rate drops below 20 percent or your bounce rate climbs above 3 percent, something is broken in the infrastructure, and no rewritten subject line repairs that.
We run around 170 mailboxes for ongoing client campaigns. Below is what we encounter in practice, in order of how often it is the cause.
1. Your list is contaminated
Every address that does not exist is a vote against your sending reputation. Mail providers measure how often your domain tries to deliver to addresses that are not there, and that weighs heavily.
What we see in practice: lists that come out of an export and have not been cleaned contain 5 to 15 percent dead addresses as standard. Companies that no longer exist, people who left two years ago, addresses derived from a pattern that does not apply at that company.
What you need to do, in this order:
- Validate every address before sending. Not a sample, all of them.
- Put catch-all domains in a separate segment and measure their bounce separately. Throwing them away immediately is a waste.
- Build a suppression list with defunct companies and hard bounces, and let it work across all your campaigns. Otherwise you will buy the same dead addresses again six months from now.
A practical number to steer on: below 2 percent bounce is healthy, above 3 percent you have to stop and clean the list, and above 5 percent you are burning your domains.
2. You send too much per mailbox
This is the mistake that becomes irreparable fastest. A mailbox sending 200 emails a day is a spam signal. Twenty mailboxes each sending ten are not, even though the same volume goes out.
Our standard: a maximum of 25 emails per mailbox per day. So 500 emails a day means 20 mailboxes, 1,000 a day means 40 to 80. That is why serious cold email is more expensive than people expect: you are not buying software, you are buying sending capacity.
Permanent warmup is part of that: mailboxes that keep mailing each other and a network, even while the campaign is already running. We set that at around 25 warmup emails per day per box with a response rate of about 30 percent, and we never switch it off.
3. You are mailing from your main domain
Do not do this. If something goes wrong, and at some point something will, you lose delivery of your invoices, your quotes and your client correspondence along with your campaign.
The right setup is a series of separate sending domains that resemble your main domain but are not it, each with a handful of mailboxes, each with correct SPF, DKIM and DMARC records. If one domain goes down, you replace that domain and the rest keeps running.
Two practical points that are rarely written down anywhere:
- Use personal addresses, not role accounts. In our own figures, campaigns sending from a person's name structurally outperform campaigns from info@ or contact@. An email from a person reads like a human, an email from info@ reads like a system, and filters think the same way.
- Consider the receiving side. Business mail in the US and UK runs mostly on Google Workspace and Microsoft 365, and sending to predominantly Google recipients produces a different bounce pattern than sending to Microsoft recipients. If you run into that, split the campaign by receiving provider instead of pushing harder.
4. Your tracking is your biggest weakness
This is the most underestimated cause.
Open tracking. To measure an open, your software places an invisible image in the email that is loaded from an external server. That turns a short text email into an email with external content, exactly the pattern filters recognise. On top of that, the open rate you gain from it has been unreliable for years because mail providers preload images.
Our advice: switch open tracking off when you build a campaign. Note: with some sending software you can no longer change this after a campaign has been live, so it is a decision you take up front or not at all.
Custom tracking domain. If you do use tracking, it has to run through your own subdomain with a correct CNAME record. Two things go wrong there. First: the record is missing altogether, and then your links run through a shared domain whose reputation is not yours. Second, more subtle: the record is there but has never been re-verified in the software after domains were added. Put it on your monthly checklist. We find this in virtually every campaign we take over from another provider.
Links in general. Every link is a risk. A link to a page on a domain with a good reputation is fine. Three links, a tracking pixel and an unsubscribe link in a 40-word email is a spam score.
5. You scaled up too fast
A new domain with new mailboxes cannot go into production straight away. Two weeks of warmup before the first campaign email is the minimum; three is better. We switch the warmup on two weeks before the agreed go-live date as standard, even if the client starts later. And after go-live you begin at a third of the volume and build that up over two weeks.
6. You run on the same broad list for too long
This is a pattern we only saw once we had been running long enough to see it: a broad, unsegmented list makes your domain reputation drop after three to four weeks. The emails do not get worse. A broad list by definition contains many recipients for whom the offer is not relevant. They ignore, delete or mark it, and all of that counts.
The remedy is not more volume or newer copy. It is segmentation. In a campaign of over 18,000 leads, the reply rate differed between segments by a factor of three with identical copy, in the same week, from the same infrastructure. The full figures are in how many replies do you get from 30,000 emails.
What to check every month
| Signal | Healthy | Action needed |
|---|---|---|
| Bounce rate | < 2 % | > 3 %: stop and clean the list |
| Open rate (if measured) | > 35 % | < 20 %: check the infrastructure |
| Spam complaints | ± 0 | > 0.1 %: scale back immediately, never reach 0.3 % |
| Mailbox health score | 90 to 100 | < 90: take the box out of rotation |
| Volume per mailbox | ≤ 25 per day | Higher: reduce |
| SPF / DKIM / DMARC | All three valid | Missing: fix immediately |
| Tracking CNAME | Verified | Unverified: verify again |
| Leads on ‘not started’ | Falling | Rising: the list or the capacity is wrong |
The spam complaint threshold follows Google's email sender guidelines, which ask senders to keep their user-reported spam rate below 0.1 percent and never let it reach 0.3 percent. Gmail and Google Workspace handle a large share of business mail in the US and UK, so this is the bar to plan for. Accessed 4 October 2026.
That last line is one almost nobody measures and it costs a lot of money. If there are thousands of leads in your campaign that have never been contacted, you are paying for volume that does not go out. Ask your supplier every month for the actual number of emails sent, not just for the number of replies.
What does not help
- Rewriting the subject line when your bounce rate is 6 percent.
- More volume to compensate for a disappointing campaign. All you do is speed up the damage.
- A spam checker score. It says something about your copy, nothing about your reputation, and reputation is the largest part of the problem.
- Waiting. A declining reputation does not recover by itself while you keep sending.
Frequently asked questions
What is a good bounce rate for cold email?
Below 2 percent is healthy. Above 3 percent you have to stop sending and clean the list. Above 5 percent you are damaging your sending domains and it is cheaper to start over than to push on.
How many emails per mailbox per day can I safely send?
A maximum of 25. That is the standard we maintain across around 170 mailboxes. If you want to send 1,000 emails a day, you therefore need 40 to 80 mailboxes. Volume comes from the number of mailboxes, not from harder settings on a single box.
Should I switch on open tracking for cold email?
Preferably not. Open tracking adds an externally loaded image to your email, and that is exactly the pattern spam filters recognise. The figures you gain from it are also unreliable because mail providers preload images. Decide it before go-live: with some sending software you cannot switch it off afterwards.
Can I send cold email from my own company domain?
Technically yes, sensibly no. If the reputation goes down, you lose delivery of your invoices, quotes and client mail along with your campaign. Use separate sending domains with their own SPF, DKIM and DMARC records, each with a handful of mailboxes.
Why does my open rate drop after three or four weeks?
Almost always because you are running on a broad, unsegmented list. Such a list contains many recipients for whom the offer is not relevant; they ignore or delete the email, and that counts towards your reputation. The solution is to segment and mail a different angle per segment, not new copy on the same list.
Is a campaign of yours failing to arrive?
We will look at it free of charge: infrastructure, records, volume per mailbox and list quality. Also if you are with another provider, then at least you know what is broken.
Rather email first? That works too: info@link2leads.nl