Deliverability isn’t one setting you flip. It’s the sum of a dozen signals that mailbox providers weigh every time you send, and the weighting shifts constantly. But the signals themselves are stable and well understood. Get these right and you’ll spend far less time wondering why a campaign underperformed. A blunt truth up front: a clean IP and passing SPF are necessary but nowhere near sufficient — the biggest levers are usually list quality and engagement, not DNS.
1. Authenticate — and align
Non-negotiable and first. SPF and DKIM valid, DMARC published and moving to enforcement, and — the part people miss — aligned with your From: domain. Since 2024, Gmail and Yahoo require this from bulk senders; see why newsletters fail. Confirm the real result with the Outbound Deliverability Test.
2. Protect your sending identity
- Reverse DNS (PTR) on the sending IP, forward-confirmed. Many receivers reject mail without it — check it.
- Consistent HELO/EHLO hostname that matches your sending infrastructure.
- Not on blocklists — check the sending IP, not just your inbound MX (Blacklist Checker).
- TLS on delivery — modern providers expect it.
3. Send to good addresses only
This is the single most underrated lever. Mailing invalid addresses drives up bounces; mailing recycled spam traps torches your reputation directly. Before any significant send, run your list through the Email Verifier (or the bulk API) to strip invalid, disposable and role addresses, and be cautious with catch-all domains where acceptance doesn’t prove a mailbox exists. Purge chronically unengaged contacts on a schedule — a smaller engaged list beats a big stale one every time.
4. Warm up and keep volume steady
New IPs and domains have no reputation, so providers are suspicious of sudden volume. Ramp gradually over days or weeks, and avoid spiky sending patterns. A steady, predictable cadence reads as legitimate; a burst reads as a compromised account.
5. Earn engagement, avoid complaints
- Opens, clicks and replies are positive signals; deletions-without-open and “mark as spam” are strongly negative.
- Keep the spam-complaint rate low — Google targets under 0.3%, ideally below 0.1%.
- One-click unsubscribe (RFC 8058) is now mandatory for bulk senders and, counter-intuitively, protects you: an easy unsubscribe is far better for reputation than a spam complaint. Verify it with the List-Unsubscribe Checker.
- Only send to people who asked. Confirmed (double) opt-in pays for itself in deliverability.
6. Get the content basics right
- Send multipart/alternative — a plain-text part alongside the HTML.
- Avoid image-only emails and a heavy image-to-text ratio.
- Keep link domains reputable and consistent; a shady link domain can get the whole message blocked.
- Include a real physical address and a visible unsubscribe link (also a legal requirement in many regions).
7. Monitor, don’t guess
Reputation decays quietly. Watch DMARC aggregate reports to catch a misconfigured or spoofed sender early; keep an eye on provider postmaster tools (Google Postmaster Tools, Microsoft SNDS); and re-test after any change to your sending setup. When something slips, the Outbound Deliverability Test tells you what your live mail is actually doing.
The one-paragraph version
Authenticate and align SPF/DKIM/DMARC; put PTR on your sending IP and stay off blocklists; verify your list and mail only engaged, opted-in people; warm up and send steadily; make unsubscribing trivial and complaints rare; send proper multipart content; and monitor your reports. Do those and you’re doing better than most senders on the internet. Start with the Email Health Check and the health cheat sheet for the DNS foundation.