Deliverability is the percentage of your emails that reach the inbox instead of the spam folder or being silently dropped. It's the single most important variable in cold email — a 10% response rate means nothing if 60% of your emails never arrived.
This guide covers everything you need to build a deliverable cold email infrastructure from scratch. No surface-level tips — just what actually matters and why.
Gmail, Outlook, and other inbox providers use a combination of signals to decide whether to deliver your email. These signals fall into three categories:
Most deliverability problems come from one of three root causes: missing or misconfigured DNS records, sending too much too fast from a cold domain, or writing emails that look like spam.
The 2024 Gmail and Yahoo requirement: Since February 2024, Google and Yahoo require bulk senders (1,000+ emails/day to Gmail addresses) to have SPF, DKIM, and DMARC all correctly configured. Senders without DMARC authentication are rejected outright. This is no longer optional.
These three DNS records form the authentication foundation of email deliverability. All three must be in place before any serious cold outreach.
SPF specifies which mail servers are authorized to send email on behalf of your domain. It's a TXT record in your DNS that lists the IPs allowed to send from your domain. If an email arrives from an IP not on that list, the receiving server knows it's unauthorized.
A basic SPF record for Google Workspace looks like: v=spf1 include:_spf.google.com ~all
Common SPF mistake: Using +all (permit all) instead of -all or ~all. Also: you can only have ONE SPF record per domain. Multiple SPF TXT records break authentication.
DKIM adds a cryptographic signature to every outgoing email. The receiving server looks up a public key in your DNS and uses it to verify the signature hasn't been altered in transit. It's the strongest deliverability signal of the three — Gmail heavily weights DKIM when calculating sender reputation.
Your email sending platform (Google Workspace, Smartlead, Instantly, etc.) will generate a DKIM key for you. You paste the public key into DNS as a TXT record. The format is typically: selector._domainkey.yourdomain.com
Use 2048-bit keys, not 1024-bit. Google requires 2048-bit for Workspace.
DMARC tells receiving servers what to do with emails that fail SPF or DKIM checks. It also enables reporting — you can get daily digest emails showing authentication failures and any spoofing attempts targeting your domain.
| DMARC Policy | What it does | When to use |
|---|---|---|
p=none | Monitor only — emails still delivered even if auth fails | Starting point while setting up |
p=quarantine | Failed emails go to spam folder | After confirming SPF/DKIM are working |
p=reject | Failed emails are rejected outright | Fully configured, mature sending domains |
Minimum viable DMARC record: v=DMARC1; p=none; rua=mailto:dmarc@yourdomain.com
The rua tag sends aggregate reports to your email. Read them — they show you when authentication is failing and from where.
Email warmup is the process of building a sending reputation for a new domain or mailbox before using it for cold outreach. Inbox providers look at historical sending behavior — a domain that has only ever sent 10 emails/day and then suddenly sends 2,000 in 24 hours looks like a compromised account or a spam operation.
Warmup tools (built into Smartlead, Instantly, and others) work by sending emails between a network of seed accounts that automatically open, reply to, and un-spam your emails. This signals to inbox providers that people want to receive mail from your domain.
| Week | Daily send volume | What to do |
|---|---|---|
| Week 1 | 10–20/day | Warmup only — no cold outreach |
| Week 2 | 20–40/day | Warmup only — monitor Gmail Postmaster |
| Week 3 | 40–80/day | Can begin very limited cold outreach (20–30 emails) |
| Week 4+ | Ramp 20–30% per 3 days | Scale cold outreach alongside warmup |
Keep warmup running permanently. Most tools recommend running warmup even at full capacity — it provides a buffer of positive engagement that offsets any spam complaints from cold email. Never turn it off.
Brand new domains (registered today) are highest risk. Google treats them with deep suspicion. Best practice for agencies and high-volume senders:
The general recommendation is 2–3 mailboxes per domain, and no more than 30–50 emails per mailbox per day for sustainable cold outreach. If you need to send 1,000 emails/day, you need roughly 20–33 mailboxes across 7–10 domains.
| Sends/day target | Mailboxes needed | Domains needed |
|---|---|---|
| 100 | 3–4 | 2 |
| 500 | 15–20 | 6–8 |
| 1,000 | 30–35 | 10–12 |
| 5,000 | 150+ | 50+ |
If you're using click or open tracking, set up a custom tracking subdomain (e.g., track.yourdomain.com) rather than the default tracking domain from your ESP. Shared tracking domains accumulate spam reports from other users on the platform, dragging your deliverability down.
Since February 2024, Gmail requires bulk senders to support one-click unsubscribe via List-Unsubscribe headers. Most modern sending platforms add this automatically. Verify yours does.
Your DNS records can be perfect and your warmup complete — but how you send matters just as much.
Google's public guidance: keep your spam rate below 0.10%. If it exceeds 0.30%, Gmail begins delivering your emails to spam. These numbers are measured in Gmail Postmaster Tools, which you should have set up for every sending domain.
Free tool from Google. Shows your domain reputation (bad/low/medium/high), spam rate, authentication results, and delivery errors. Set it up for every domain you send from. Go to postmaster.google.com and verify ownership via DNS TXT record.
Microsoft's equivalent for Outlook/Hotmail. Register at sendersupport.olc.protection.outlook.com/snds. Shows your IP reputation and complaint rates for Microsoft mailboxes.
Tools like GlockApps, Mail-Tester, and Litmus's Spam Testing let you send a test email and see where it lands across multiple inbox providers. Run a test before any major campaign. Mail-Tester.com is free for occasional use.
Check your sending IPs against blacklists at MXToolbox.com. A blacklist appearance means you need to stop sending, investigate the cause, and apply for removal. Smartlead's SmartDelivery add-on includes automated blacklist monitoring.
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Written by
Scott Holmes
AI systems consultant and cold email infrastructure specialist. Founder of Pinnacle Tech Projects. Has built and managed cold email infrastructure for agencies sending 500K+ emails/month.