Cold Email Subject Lines That Actually Get Opened
Five subject line formats that consistently produce 40 to 60 percent open rates when your infrastructure is clean.
Read article →If your open rates are sinking and replies have dried up, the problem usually isn't your message, it's your deliverability. In 2026, mailbox providers like Google and Microsoft are stricter than ever about authentication and sender reputation. Get the infrastructure right and average copy still books meetings. Get it wrong and the best copy in the world lands in spam.
1. Separate your sending domains
Never send cold email from your primary brand domain. One spam complaint shouldn't be able to torch your main inbox. Instead, register dedicated lookalike domains (for example, get-yourbrand.com or tryyourbrand.com) purely for outreach.
- Buy 3–10 sending domains depending on volume.
- Run 2–3 mailboxes per domain.
- Redirect each sending domain to your main site so it looks legitimate.
2. Authenticate everything: SPF, DKIM, DMARC
These three DNS records are non-negotiable. SPF declares which servers may send for your domain. DKIM signs each message so providers can verify it wasn't tampered with. DMARC tells inboxes what to do when checks fail. Without all three, modern filters quietly bin you.
If you only fix one thing this week, set up DMARC. It's the signal inboxes trust most in 2026.
3. Warm up before you send
A brand-new mailbox blasting 50 cold emails on day one is the fastest route to the spam folder. Warmup gradually ramps volume and simulates genuine engagement so providers build trust in your sender reputation.
- Run automated warmup for 2–3 weeks per mailbox.
- Start at ~5 sends/day and ramp to 30–40.
- Keep a portion of warmup traffic running permanently.
4. Respect daily send limits
Each mailbox should send 20–40 real cold emails per day, max. Need more volume? Add mailboxes and domains, don't push a single inbox past what looks human. Volume comes from horizontal scale, not from hammering one address.
5. Don't forget the other channels
Email is one lane. WhatsApp and LinkedIn are increasingly where replies actually happen, but they have the same reputation rules. That's exactly why we built Wassuply, our WhatsApp warming tool, to protect deliverability across multichannel outreach.
Frequently asked questions
How many domains do I need for cold email?
Use separate sending domains from your main brand domain, roughly one domain per 2–3 mailboxes, each sending 20–40 emails/day after warmup. Scale domains with volume rather than overloading one.
How long does email warmup take?
Plan for 2–3 weeks of automated warmup before real campaigns. It gradually increases volume and engagement so providers build trust in your sender reputation.
What's the difference between SPF, DKIM and DMARC?
SPF authorises sending servers, DKIM signs your emails, and DMARC tells inboxes what to do when those checks fail. All three are required for reliable deliverability in 2026.