A cold email deliverability checklist that actually reflects 2026
Sending domains, authentication, warm-up, volume, list hygiene and content — the checks that keep outbound out of spam, in the order they matter.
20 Oct 2025 · 2 min read · by HeiSyn
Deliverability is not a setting you switch on; it is the accumulated reputation of the domains and inboxes you send from, and it is easier to lose than to earn. Most outbound programmes that 'stopped working' did nothing wrong in their copy — they burned their sending reputation with volume and bad addresses. This is the order to check things in.
Domains and authentication
Never send cold email from your primary domain. Buy two or three lookalike domains, point them at your main site, and set up SPF, DKIM and DMARC on each — DMARC with a policy, not just a monitoring record, because mailbox providers now require it. Set up a custom tracking domain if you use open tracking at all; better, do not track opens, which are unreliable and add a link that filters dislike.
Warm-up and volume
New inboxes need three to four weeks of warm-up before any campaign, and warm-up should continue at a low level while you send. Keep each inbox under thirty to fifty cold emails a day, spread through working hours with random gaps. If you need more volume, add inboxes and domains; do not push a single inbox harder.
List hygiene
Verify every address before it enters a sequence, and remove catch-all domains unless you can validate them another way. Bounce rate above two percent is a warning; above five percent is a stop. Suppress anyone who unsubscribed or complained, forever, across every domain you own.
Content and behaviour
Plain text, no images, one link at most and preferably none in the first email. Short. No spam-trigger words, no fake 'Re:' subjects, no tracking pixels. Personalised first lines help deliverability indirectly because they raise replies, and replies are the strongest positive signal a mailbox provider sees. Monitor daily: bounce, complaint and reply rates per inbox, and pause any inbox that drifts.
Questions people ask next
- How do I know my emails are landing in spam?
- Seed accounts at the major providers that receive every campaign and report placement, plus Google Postmaster Tools for domain reputation. Falling reply rates with stable list quality are the practical symptom; a sudden drop across all inboxes on one domain usually means the domain is being filtered.
- Can I recover a burned domain?
- Sometimes, slowly: stop sending, fix authentication, remove the bad addresses, then warm up again over four to six weeks at low volume with engaged recipients. Often it is cheaper to retire the domain and start a new one correctly. The main lesson is to keep cold email off any domain you cannot afford to lose.
- Do we need DMARC at enforcement, or is monitoring enough?
- Enforcement — a quarantine or reject policy — is what the major mailbox providers now expect from senders of any volume, and it is what stops others spoofing your domain. Start at monitoring for a week to catch legitimate senders you forgot about, then move to enforcement.
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