Email deliverability is the difference between a campaign that reaches the inbox and one that lands in spam with a respectable-looking open rate of zero. Most teams treat it as a technical checkbox owned by whoever set up DNS. In practice it is an operating discipline: authentication, list hygiene, sending behaviour, and engagement all feed the same reputation system that decides your fate before anyone reads a subject line.

This guide covers the fixes that matter most, in the order that produces results.

Start with authentication: SPF, DKIM, DMARC

Three records do most of the work, and all three are non-negotiable if you send at volume.

  • SPF lists the servers allowed to send for your domain. Keep it under ten DNS lookups; bloated SPF records silently fail.
  • DKIM signs each message cryptographically, so receivers can verify it was not altered in transit. Use a 2048-bit key and rotate it periodically.
  • DMARC ties the two together and tells receivers what to do when authentication fails. Start with p=none and a reporting address, read the reports for two weeks, then move to quarantine and eventually reject.

Also align your sending domain: if the visible From address is your brand but the mail is sent through a third party, the domains must align or DMARC will fail. This single mistake accounts for a large share of unexplained spam placement.

Separate your streams

Transactional mail — receipts, password resets, shipping updates — should not share a domain or IP reputation with marketing campaigns. If a promotional blast tanks, you do not want password resets going down with it. Use subdomains: mail.yourbrand.com for marketing, notifications.yourbrand.com for transactional.

The same logic applies to cold outreach. Never send cold email from your primary domain; use a separate one, and accept that it may burn.

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List hygiene is not optional

Purchased lists are the fastest route to a spam folder. Beyond that, the everyday hygiene rules:

  1. Remove hard bounces immediately. They are permanent; retrying them is pure reputation damage.
  2. Retire chronic non-openers. If someone has not opened in nine to twelve months, stop mailing them or run a deliberate re-engagement sequence and remove the rest.
  3. Validate at capture. Double opt-in where consent rules require it, plus syntax and domain checks before the address enters the list.
  4. Watch spam complaints. Above roughly 0.1% of delivered mail, providers start throttling. Above 0.3%, you have a problem that needs a fix, not a new campaign.

Warm up new domains and IPs slowly

A new sending domain has no reputation. Sending 50,000 messages on day one looks exactly like a spammer’s behaviour. Start with your most engaged segment — a few hundred recipients — and double volume every few days while watching bounce and complaint rates. Two to four weeks of gradual ramp is normal.

The same applies after a long pause. If you have not mailed in six months, you are effectively new again.

Content signals that still matter

Filters are less keyword-driven than they used to be, but several content habits still hurt:

  • Image-only emails. A message with no text and one big picture is a classic spam shape. Keep a healthy text-to-image ratio.
  • Link shorteners and mismatched display URLs. Link to your own domain, plainly.
  • Attachment-heavy campaigns. Use a landing page, not a PDF.
  • Repeated near-identical sends. Vary content between campaigns; identical bulk mail is a pattern.

Plain, well-structured HTML with a real unsubscribe link and a physical address (where required) beats clever design every time.

Measure what receivers measure

Track these weekly, per stream:

  • Bounce rate — hard and soft separately.
  • Complaint rate — the single most damaging metric.
  • Inbox placement — from a seed-list tool, split by provider (Gmail, Outlook, Yahoo).
  • Engagement — opens and clicks, interpreted as a reputation signal rather than a KPI.
  • DMARC pass rate — any sustained dip means someone is sending on your behalf.

Check Google Postmaster Tools and Microsoft SNDS if you send at volume; both are free and show reputation directly.

When you are already in spam

Diagnose before changing anything. Confirm authentication passes, check whether the problem is one provider or all of them, and look at recent sending changes — new list, new domain, volume spike, template change. Then cut volume to your engaged segment, fix the technical fault, and rebuild gradually. Reputation recovers in weeks, not hours, and only if the underlying cause is gone.

Common mistakes

Buying a list “just to test”. Reputation damage outlives the test.

Sending to everyone every week. Frequency without segmentation trains people to ignore you and providers to filter you.

Ignoring the reply-to and unsubscribe. A one-click unsubscribe that works is a spam complaint avoided.

Treating deliverability as a one-off setup. It is a monthly review, not a migration project.

Deliverability is unglamorous and mostly invisible when it works — which is exactly why it deserves a standing slot in your operations calendar rather than a rescue mission after a bad send.

Once mail lands, the next question is what it should do: our guide to email marketing automation covers the lifecycle flows worth building, and customer retention strategies explains how those flows fit the bigger picture.

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