Email deliverability is whether a message reaches the recipient’s main inbox rather than being blocked, bounced or routed to spam, and it depends on several things working together. The fastest fixes are getting authentication (SPF, DKIM and DMARC) properly aligned, removing bad addresses from your list, and watching the signals that mailbox providers and M3AAWG use to judge senders through tools like Google Postmaster Tools.
Table of Contents
- A quick checklist to triage delivery problems right now
- Why SPF, DKIM and DMARC matter and how to check they are correct
- Keeping your list clean and handling bounces properly
- Volume, cadence and warming up a new sending IP
- How content and recipient behaviour shape inbox placement
- Which tools to monitor and how to read bounce codes
- When it makes sense to bring in a deliverability specialist
- FAQ
- Sources
A quick checklist to triage delivery problems right now
Before you dig into logs or call anyone in, run through this sequence. This sequence will usually tell you whether you have a technical fault, a reputation problem, or both.
- Run an authentication check to confirm SPF, DKIM and DMARC all pass and align with your sending domain.
- Pull your recent bounces and NDRs, then suppress every hard bounce immediately rather than retrying.
- Check Google Postmaster Tools or your ESP dashboard for spam rate and reputation flags from the last 48 hours.
- Verify your List-Unsubscribe header is present and that clicking unsubscribe actually removes the recipient.
- Look for a sudden spike in sending volume or a shared IP shared with poor-quality senders, and pause campaigns if either shows up.
Pro Tip: If you recently changed ESP, sending domain or IP, treat that as your prime suspect before looking anywhere else.
Why SPF, DKIM and DMARC matter and how to check they are correct
Authentication is the part mailbox providers check first, and it is also where most senders get something subtly wrong. Microsoft’s own troubleshooting guidance confirms that SPF, DKIM, DMARC, sending reputation and complaint rates all feed into how Microsoft 365 filters incoming mail, alongside the NDRs and SMTP responses you should be collecting as a matter of course.
- SPF needs to list every legitimate sending source without exceeding the 10 DNS lookup limit, which is easy to breach once you add a third ESP or CRM.
- DKIM keys should validate cleanly on every message, and 2048-bit keys are worth using wherever your platform supports them.
- DMARC is safest to roll out gradually: start at p=none to gather reports, then move to quarantine and eventually reject once you have confirmed every authorised sender.
- Alignment matters as much as the records themselves: your visible From domain must match either the SPF or DKIM domain, or DMARC has nothing to enforce.
- Missing reverse DNS (PTR) records or weak TLS support can get you rejected or throttled before content is even scanned.
Our SPF, DKIM and DMARC explainer goes through the DNS changes in more depth if you need to make the edits yourself.
Keeping your list clean and handling bounces properly
List quality sits right behind authentication in terms of impact, and it is largely within your control. M3AAWG’s sender best practices treat confirmed opt-in as the standard to aim for, because it filters out invalid addresses and typo domains before they ever reach your sending list.
- Favour confirmed (double) opt-in over single opt-in wherever your sign-up flow allows it.
- Suppress hard bounces the moment they occur and watch soft bounces for repeating patterns on the same domain.
- Reconfirm long-term inactive subscribers before removing them, rather than letting them quietly drag down engagement rates.
- Add List-Unsubscribe headers following RFC 8058 and process unsubscribe requests within a day or two.
- Never buy or rent a list, and strip out obvious role addresses and misspelt domains before your first send.
Volume, cadence and warming up a new sending IP
Reputation is built gradually rather than earned in a single good week, which is why ramping volume carefully matters more than most senders expect. Start low, increase gradually, and spread sends across the day rather than firing one large batch the moment your warm-up period ends.
- Keep transactional mail on a separate IP or stream from marketing campaigns, so a bad campaign never threatens password resets or receipts.
- Watch for deferrals and 4xx or 5xx SMTP responses during warm-up and slow your pace the moment they appear.
- Shared IPs can work in your favour or against you depending entirely on who else is using them, which is why high-volume, consistent senders tend to move to a dedicated IP.
- Full warm-up typically takes several weeks, and recovering a reputation damaged by abusive sending tends to take considerably longer than building one from scratch.
Pro Tip: If you are validating new send times or cadence changes across a large list, a structured test such as the 200-contact send time test gives you a cleaner read than guessing from aggregate open rates.
How content and recipient behaviour shape inbox placement
Filters read your message much the way a sceptical recipient would, and the signals that trip them are largely avoidable with a bit of discipline. Deceptive subject lines, excessive punctuation, blocks of capital letters and image-only emails with barely any real text all raise flags that content filters are specifically built to catch.
- Write subject lines that describe the content honestly rather than manufacturing urgency with exclamation marks or all capitals.
- Keep a healthy ratio of readable text to images, and avoid stacking multiple redirects behind a single link.
- Send to your most engaged segments first, since opens, clicks and replies all feed positively into how mailbox providers score you.
- Test subject lines and preview text in small batches before a full send, because small wording changes often move engagement more than you would expect.
Which tools to monitor and how to read bounce codes
Diagnosing a deliverability problem is largely a matter of reading the right signals in the right order, rather than guessing. Gmail’s sender guidelines recommend keeping spam complaint rates below 0.1% and treat 0.3% as a hard threshold, alongside requirements for valid SPF or DKIM, correct forward and reverse DNS, and TLS on every connection.
- Check Google Postmaster Tools, Yahoo’s Sender Hub and your ESP’s own delivery logs for spam rates, authentication failures and domain or IP reputation scores.
- Collect full SMTP responses and NDRs, and use the status code class to distinguish success from temporary and permanent failures, then decide whether to retry or suppress an address.
- Check your sending IP and domain against major blacklists, and confirm reverse DNS and TLS are both configured correctly.
- If a specific mailbox provider is marking legitimate mail as spam, use its own escalation route, such as Microsoft’s junk mail reporting process or Gmail’s feedback loop, rather than simply resending.
- When those internal checks are exhausted and the problem persists, a specialist deliverability review tends to find the systemic cause faster than continued trial and error.
For Microsoft-specific recovery timelines, our guide on recovering Outlook deliverability sets out what a realistic 48 to 72 hour fix looks like.
When it makes sense to bring in a deliverability specialist
If authentication keeps failing, complaint rates have spiked, or inbox placement has stayed poor for weeks despite following the steps above, a professional audit will usually resolve things faster than continued ad hoc fixes. We work through a straightforward path: a free health check, then a full deliverability review, then the actual remediation work on authentication, warm-up or infrastructure, followed by ongoing monitoring so the fix holds.
- We carry out the technical work ourselves rather than only reporting what is wrong.
- Our free health check gives you a starting diagnosis before any commitment.
- A chargeable advisory session (£250 one-off) suits teams who want a rapid, expert read on a specific problem.
If you recognise your situation in any of the sections above, start with our deliverability services page to see the free health check and review options and get a diagnosis moving.
FAQ
What is deliverability in email?
Email deliverability is whether a message successfully reaches the recipient’s inbox rather than being blocked, bounced, or filtered into spam. It depends on authentication, sender reputation, list quality and content, all of which mailbox providers assess before deciding where to place a message.
How do I fix email deliverability problems?
Start by confirming SPF, DKIM and DMARC are correctly set up and aligned, since Microsoft’s guidance and most mailbox providers check these first. Then clean your list of hard bounces and inactive addresses, and monitor spam and complaint rates through tools such as Google Postmaster Tools.
What is the 30/30/50 rule for cold emails?
This isn’t a standard mailbox-provider or industry rule, and no authoritative source defines it consistently, so treat any version you encounter with caution. The reliable guidance instead comes from recognised sources such as M3AAWG and mailbox providers’ own sender guidelines, which focus on consent, authentication and sending reputation rather than fixed percentage splits.
What is a good email deliverability rate?
Gmail’s sender guidelines recommend keeping spam complaint rates below 0.1%, with 0.3% treated as a hard ceiling before mitigation eligibility is affected. Bounce rates should stay low and consistent, and a sudden rise in either figure is usually the clearest early sign of a developing problem.
Sources
- M3AAWG Sender Best Common Practices Aug-27-2026
- Email sender guidelines - Gmail Help
- Troubleshooting mail sent to Microsoft 365 - Microsoft Defender for Office 365 | Microsoft Learn
- The Complete Email Deliverability Checklist for 2026 (+Tools & Tips) | WordStream
