Cold Email Deliverability: A Practical Checklist
Cold outreach lives or dies on deliverability. Here's the checklist that keeps you in the inbox: authentication, warmup, volume, and clean lists.
Overview
Cold email deliverability is the difference between landing in the inbox and never being seen. It's mostly won before you write a word: an authenticated domain, a warmed-up mailbox, conservative volume, and a clean, verified list. Mailbox providers like Gmail and Outlook judge you on authentication, complaint rate, and engagement — get those right and good copy can do its job; get them wrong and the best email in the world sits in spam.
The foundations to set up before sending one email
Cold deliverability is won in setup, not copy. Do these in order: 1) Buy a separate sending domain for cold outreach (a look-alike of your main one) so any damage stays off your primary domain. 2) Authenticate it — SPF, DKIM, and DMARC all configured and passing; Google and Yahoo now effectively require this for bulk senders. 3) Create a few mailboxes on it rather than one. 4) Warm each mailbox up for 2–4 weeks before real volume. 5) Verify your list so bounces stay near zero from day one.
A workable warmup and volume ramp for a fresh mailbox: week 1 ~5–10 emails/day, week 2 ~15–20, week 3 ~25–30, week 4+ ~40–50 and hold. Cap each mailbox around 40–50 cold sends/day even once warm — to scale, add mailboxes (each warmed), don't push one inbox harder. A brand-new domain blasting hundreds a day is the single clearest spam signal there is.
- Separate, look-alike cold-outreach domain — never your main one
- SPF, DKIM, and DMARC configured and passing
- Warm each mailbox 2–4 weeks; ramp 10 → 50/day
- Cap ~40–50 cold sends/mailbox/day; add mailboxes to scale
- Verify the list before the first send
Staying in the inbox over time
Once you're sending, watch the two metrics providers care about most: bounce rate (keep it under ~2% by verifying before every send) and spam-complaint rate (keep it under ~0.3% — Google's stated threshold; above it, deliverability collapses fast). Honor unsubscribes instantly, because someone who can't opt out hits "report spam" instead, and that's far more damaging than losing the prospect. Maintain a suppression list so opted-out and bounced addresses are never re-contacted.
On content, look like a person, not a campaign: plain-text-style emails, minimal links (ideally one), no heavy images or link shorteners, and a real reply-to. Engagement is itself a signal — replies and "not spam" moves lift reputation, while deletes-without-reading sink it, so relevance beats volume. For the legitimate email your business sends — campaigns, transactional, re-engagement — Mailbase enforces suppression at send time and surfaces bounce/complaint clustering and domain health, so the reputation you build with good habits doesn't get quietly eroded. See also how to avoid the spam folder.
- Bounce rate under ~2%; complaint rate under ~0.3%
- Honor unsubscribes instantly; maintain a suppression list
- Lean, link-light, plain-looking emails with a real reply-to
- Relevance drives engagement, which drives reputation
Common Mistakes
- Following up with “just bumping this” instead of adding a new angle.
- Measuring opens (inflated by proxies) instead of replies and meetings.
- Skipping warmup and blasting a new domain at full volume.
- Letting replies scatter across reps' inboxes so leads get dropped or double-touched.
Sources & Further Reading
Official docs for current setup details, pricing, and API behavior — verify specifics there, since they change.
Related guides
More on cold email deliverability and the surrounding email outreach workflow:
FAQ
Why do my cold emails go to spam?
Usually a mix of weak authentication (missing or failing SPF/DKIM/DMARC), a cold un-warmed domain, too much volume too fast, an unverified list causing bounces, or spammy content (all-image, link-heavy). Fix authentication, warmup, and list hygiene first — copy is the smallest factor.
How do I warm up a domain for cold email?
Send small and ramp slowly from a new mailbox: roughly 5–10 emails/day in week 1, building to 40–50/day by week 4, ideally to engaged recipients who reply. Many senders use a warmup tool to automate early back-and-forth. Never jump a fresh domain straight to real volume.
What bounce and complaint rates are safe?
Keep bounce rate under about 2% (verify before sending) and spam-complaint rate under about 0.3% — Google's published threshold for bulk senders. Crossing the complaint line is one of the fastest ways to wreck a domain's deliverability.
Does cold email hurt my main domain's deliverability?
It can, which is exactly why you send cold outreach from a separate look-alike domain. That keeps any reputation damage away from the domain that delivers your transactional and product email.