Deliverability feels mysterious because the mailbox providers do not explain their decisions. In practice it is not mysterious at all: they are asking whether you are who you claim to be, and whether the people receiving your mail want it.
Get authentication right, keep complaints low and stop mailing people who never engage. That is the whole discipline, and the order matters because the first item is a one-off technical job and the other two are habits.
Quick Info
First thing to check
SPF, DKIM and DMARC on your sending domain
Complaint rate to stay under
0.1%, ideally 0.05%
Bounce rate to stay under
2%
Most common cause of a sudden drop
Sending to an old, unengaged list
Recovery time from damaged reputation
Four to twelve weeks
What matters least
Spam-trigger words
1. Authenticate the domain properly
Mailbox providers now enforce authentication for bulk senders. Without it, a proportion of your mail is filtered or rejected regardless of content, and there is no content fix for a missing DNS record.
01SPF: a DNS record listing the servers allowed to send for your domain. Your email platform gives you the value.
02DKIM: a cryptographic signature proving the message was not altered and came from you. Also a DNS record from your platform.
03DMARC: a policy telling providers what to do when SPF or DKIM fail. Start at p=none to collect reports, then move to quarantine once you know what is sending on your behalf.
04Use a subdomain for marketing mail, such as mail.yourdomain.co.uk, so a marketing reputation problem does not damage your business email.
05Verify all three with a checker after setup. Do not assume the platform did it.
This is twenty minutes with your DNS and it is the single highest-return item in email. It is also the thing most commonly half-done: SPF set, DKIM never verified.
2. Keep the complaint rate down
The complaint rate, people marking you as spam, is the metric that most directly damages your reputation. The major providers publish a 0.1% threshold and act well before you reach it.
—Make the unsubscribe link obvious. A visible one-click unsubscribe reduces complaints, because the alternative to unsubscribing is reporting you.
—Send what you promised at signup, at the frequency you promised. Complaints spike when the mail is a surprise.
—Never mail people who did not opt in. Bought lists, scraped addresses and conference badge scans all produce complaints immediately.
—Segment. Sending irrelevant email to the wrong half of your list is the most common cause of a creeping complaint rate.
—Watch it per campaign, not just overall. One bad send can undo three months of good behaviour.
Every spam complaint is a vote counted against every future email you send. It is much cheaper to lose a subscriber than to earn a complaint.
3. Clean the list
Mailing addresses that never open teaches providers that your mail is unwanted. Engagement is now a primary filtering signal, so a large unengaged segment actively harms delivery to the people who do want you.
01Remove hard bounces immediately. Any decent platform does this automatically; confirm yours is.
02Keep total bounce rate under 2%. Above that, your acquisition source is the problem.
03Suppress anybody with no opens or clicks in six months, after one re-engagement attempt.
04Never mail a list older than a year without a re-permission campaign. Old lists are the most common cause of a sudden deliverability collapse.
05Use double opt-in if you get spam signups or form abuse.
06Do not use list-cleaning services as a substitute for engagement-based suppression. They catch invalid addresses, not uninterested people.
Shrinking a list feels like going backwards and improves every metric that matters, including how many real people see your mail.
4. Warm up any new sending setup
A new domain, subdomain or platform has no reputation. Sending fifty thousand emails on day one from a cold setup is the fastest route to being filtered.
01Start with your most engaged segment: people who opened something in the last thirty days.
02Increase volume gradually over two to three weeks.
03Watch bounce and complaint rates at each step and pause if either rises.
04Keep sending consistently afterwards. Long gaps followed by a large send look like a compromised account.
5. Content, which matters less than people think
Spam-trigger word lists are largely folklore in 2026. Filters weigh sender reputation and recipient engagement far more heavily than vocabulary. That said, a few content factors genuinely matter.
—Image-only emails with no text. Filters cannot read them and engaged readers cannot either.
—Link shorteners, which share reputation with whatever else used them.
—Poor text-to-image ratio and heavy nested HTML.
—Mismatched or misleading subject lines, which produce complaints, which is the real mechanism.
—Attachments. Do not send them in marketing mail; link instead.
—Sending from a free address like gmail.com as the from-domain, which now fails authentication policies outright.
If you write plainly, from a person, with a real text version and honest subject lines, content is not your problem.
6. Monitor what you can see
01Google Postmaster Tools, for reputation, complaint rate and authentication results on Gmail traffic. Free and the most useful signal available.
02Microsoft SNDS for Outlook traffic.
03Your platform's own deliverability and bounce reporting.
04DMARC reports, which reveal anything else sending as your domain.
05Seed testing across a few real inboxes before large sends, which catches placement problems tools miss.
Open rate is not a deliverability metric any more, because privacy features distort it in both directions. Use clicks and Postmaster reputation instead.
Diagnosing a sudden drop
01Check authentication is still passing. DNS changes and platform migrations break it silently.
02Check whether the drop is provider-specific. Gmail only, or Outlook only, points at that provider's reputation signals.
03Look at the last send before the drop. Volume spike, new segment, old list, changed from-address?
04Check complaint and bounce rates on recent campaigns.
05Check whether you are on a blocklist, using a public lookup tool.
06Check whether somebody else on a shared IP caused it, if your platform uses shared sending.
In my experience the answer is one of three things: authentication broke, somebody mailed an old list, or the from-address changed. Look there first.
Recovering a damaged reputation
It is possible and it is slow. There is no appeal process and no shortcut.
01Stop sending to unengaged addresses entirely.
02Reduce volume substantially and mail only your most engaged segment for two to four weeks.
03Fix authentication completely, including DMARC.
04Increase volume gradually as reputation recovers in Postmaster Tools.
05Consider a fresh sending subdomain if the reputation damage is severe, and warm it properly.
06Change the habits that caused it. A clean recovery followed by the same behaviour lands you back in four weeks.
Reputation is earned slowly and lost in a single send. Behave as though every campaign is a deposit.
Frequently Asked Questions
Why are my emails going to spam?
In order of likelihood: authentication is incomplete, you are mailing unengaged addresses, or your complaint rate is too high. Content is rarely the cause.
What are SPF, DKIM and DMARC?
Three DNS records that prove your mail genuinely comes from your domain and tell providers what to do if that check fails. All three are effectively required for bulk sending now.
What is an acceptable complaint rate?
Under 0.1%, and ideally under 0.05%. Providers act well before you reach the published threshold.
Should I remove subscribers who never open?
Yes, after one re-engagement attempt. Unengaged addresses harm delivery to the people who do want your mail.
Do spam-trigger words matter?
Barely. Sender reputation and engagement dominate. Image-only emails, link shorteners and misleading subject lines matter far more than vocabulary.
How long does it take to fix deliverability?
Authentication fixes take effect within days. Reputation recovery takes four to twelve weeks of consistent, engaged sending.
Before You Go
Authenticate the domain, keep complaints under 0.1%, and stop mailing people who never engage. Those three cover almost every deliverability problem I have been asked to diagnose.
Check your DNS records this afternoon; it is the fastest win available in email. Segmentation is what keeps the complaint rate down long term.
Authenticate. Then stop mailing the uninterested.
WRITTEN BY TARIQ SALLAM
Marketing Consultant. Entrepreneur. Content Creator.
I'm a marketing consultant, entrepreneur and content creator. I help businesses grow through practical marketing, websites, SEO, content and AI.
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