The Complete Guide to Cold Email Deliverability (2026)

1. Why Deliverability Matters

You can write the perfect cold email, but if it lands in spam, nobody sees it.

Cold email deliverability is the foundation of B2B lead generation through outbound. Get this wrong and nothing else matters. The best copy and targeting in the world won't help you if your emails don't reach the inbox.

The deliverability chain

1. Your email leaves your mailbox
2. It passes through your email provider's servers
3. It hits the recipient's email provider
4. Their spam filters evaluate it
5. It lands in inbox, spam, or gets blocked entirely

You control steps 1 and 2. Steps 3 to 5 depend on your sender reputation, infrastructure, and content.

2. The Golden Rule: Secondary Domains

This is probably the most important thing I can tell you about cold email infrastructure: never send cold email from your main business domain.

If your domain gets flagged for spam, your entire business email is compromised. Client emails, invoices, internal communications, everything. I've seen this happen to people and it's not a fun situation to be in.

How to set up secondary domains

Buy domains specifically for cold outreach:

• yourcompany-mail.com
• getyourcompany.com
• yourcompanygroup.com
• meetyourcompany.com

Domain selection tips

• Keep them similar to your main domain (this builds trust with recipients)
• Avoid spammy words like free, best, or discount
• Use common TLDs like .com, .co, or .io
• Buy from reputable registrars like Namecheap, Cloudflare, or GoDaddy

How many domains do you need?

Plan for rotation. Domains can get burned over time, so you'll want backups ready.

Starting out: 3 to 5 domains

Scaling: 10 to 20+ domains

When one shows deliverability issues, you swap it out. This is exactly why you never use your primary domain for cold outreach.

3. Mailbox Providers for Cold Outreach

Where you buy your mailboxes matters. Different providers have different reputations with inbox providers like Google and Microsoft.

Recommended providers

1. Scaledmail
Mixed Google and Microsoft bundles. Best support in the industry. Trusted by the biggest agencies. This is my top recommendation if budget allows.

2. InboxKit
API-driven, powers a lot of the mailbox purchases you see inside sequencers. $2.50 per mailbox per month. Solid and reliable.

3. CheapInboxes
Best for beginners. Cheap Google Workspace accounts with excellent support. Great place to start if you're testing.

Google vs Microsoft

Both work well. Google Workspace has historically been the gold standard, but Microsoft 365 has caught up. Many agencies now use a mix of both for diversification.

What to avoid

• Free email providers like Gmail, Outlook.com, or Yahoo
• Shared hosting email
• Unknown providers with suspiciously low prices (these can be .edu panels or legacy Google Workspace that you want to avoid)
• Bulk email services like Mailchimp or SendGrid for cold outreach

4. How Many Mailboxes Per Domain

The safe approach is 3 mailboxes per domain.

If you're more budget-conscious, you can push to 5, but I wouldn't go beyond that. More than 5 mailboxes on a single domain starts to look suspicious to email providers. Multiple senders from the same domain all sending cold email at the same time raises red flags.

Naming conventions

Use realistic email addresses that look like they belong to real people:

Numbers in email addresses look automated. Generic handles like sales, outreach, or info tend to trigger spam filters.

The math

If you want to send 5,000 cold emails per month:

• 10 emails per mailbox per day
• Roughly 170 emails per mailbox per month
• 5,000 divided by 170 = around 30 mailboxes needed
• 30 mailboxes divided by 3 per domain = 10 domains

It's worth planning your infrastructure before you start sending.

5. Setting Up SPF, DKIM & DMARC

These three DNS records authenticate your emails and prove you're a legitimate sender. They're important for building sender reputation.

What they do

SPF (Sender Policy Framework)
Specifies which servers can send email on behalf of your domain.

DKIM (DomainKeys Identified Mail)
Adds a digital signature to your emails, proving they haven't been tampered with.

DMARC (Domain-based Message Authentication)
Tells receiving servers what to do with emails that fail SPF or DKIM checks.

My advice

Let your mailbox provider handle this.

Unless you're technical and know exactly what you're doing, I'd recommend not trying to configure these yourself. The providers I mentioned above (Scaledmail, InboxKit, CheapInboxes) will configure everything correctly for you.

If you need to set it up yourself

Basic setup:

Start with p=none (monitoring only) before moving to p=quarantine or p=reject.

6. How to Warm Up Email Domains

Email warmup builds sender reputation before you start sending cold emails. New mailboxes have zero reputation with email providers, so sending cold email immediately will likely land you in spam.

The warmup process

Week 1 to 3 (warmup only):

• Start at 2 warmup emails per day (sent and received)
• Increase by 1 email per day
• After 3 weeks, you should be at around 20 warmup emails per day

Week 4 onwards (live sending):

• Maintain 10 warmup emails per day
• Add 10 cold emails per day
• Total: 20 emails per day per mailbox

What warmup actually does

Warmup tools send emails between mailboxes in their network. These emails are automatically:

• Opened
• Replied to
• Marked as “not spam” if they land there
• Moved from promotions to primary inbox

This builds positive engagement signals with email providers, which improves your sender reputation over time.

Recommended warmup tools

Warmy.io
Expensive but solid. Dedicated warmup tool with good reputation.

Emailbison
Best built-in warmup if you're using their sequencer. They vet customers, which keeps the warmup pool clean.

Lemwarm
Good for small scale operations.

Instantly and Smartlead
Built-in warmup pools. These are a budget option, but I'd recommend monitoring them closely as quality can vary.

Warmup never stops

Keep warmup running even after you start sending cold emails. The 50/50 split (10 warmup plus 10 cold per day per mailbox) should continue for as long as you're sending.

7. How Many Cold Emails Can You Send Per Day?

The safe limit is 10 cold emails per mailbox per day.

Combined with warmup emails, your total should be around 20 emails per day per mailbox.

Why 10?

Email providers track sending patterns. A brand new domain suddenly sending 100 emails per day looks suspicious. Established senders with years of history can get away with more, but new senders can't.

10 cold emails per day per mailbox is the safe limit that most deliverability experts agree on.

Scaling up

The key is to add more mailboxes rather than increasing volume per mailbox.

Want to send 100 emails per day? Use 10 mailboxes, not 1 mailbox sending 100.

Sending patterns

• Spread sends throughout the day rather than blasting all at once
• Send during business hours in the recipient's timezone
• Randomise send times slightly (9:03am rather than exactly 9:00am)
• Avoid weekends for B2B outreach

Most sequencers handle this automatically with their scheduling features.

8. How to Verify Email Addresses

Sending to invalid addresses destroys your deliverability. High bounce rates burn through infrastructure faster than you'll see any return.

When to verify

Verify your email list immediately after purchasing data, before uploading to your sequencer. I'd recommend not trusting “pre-verified” claims from data providers. It's always worth verifying yourself.

The best email verification tools

MillionVerifier
Market favourite, reliable and affordable.

ZeroBounce
Another solid option with good accuracy and extra data enrichment features.

NeverBounce
Integrates well with most CRMs and sequencers.

What verification checks

• Valid email format
• Domain exists
• MX records configured
• Mailbox exists
• Not a spam trap
• Not a disposable address

Acceptable bounce rates

• Under 2%: Good
• 2 to 5%: Worth investigating your data source
• Over 5%: This is a serious problem, and I'd recommend stopping sending until you fix your list

How often to re-verify your list

If you're reusing lists, re-verify every month. Email addresses go stale faster than you might think. People change jobs, companies close, mailboxes get deactivated.

9. How to Handle Catch-All Emails

Catch-all emails are addresses where the domain accepts all incoming mail regardless of whether the specific mailbox exists.

For example, you might email random@company.com. If the company has a catch-all configured, it accepts the email even though “random” isn't a real mailbox.

The problem

You can't verify if the actual person exists. The verification tool sees the domain accepts mail, but has no way to confirm the specific address is real. This means you might be sending to addresses that don't belong to anyone.

My recommendation

I'd suggest avoiding catch-all emails entirely if you can. The bounce risk is too high and IT departments often flag these domains for spam.

If you need to use them

• Isolate them to a dedicated burner domain
• Run them in a separate campaign
• Expect to replace that infrastructure sooner
• Monitor bounce rates closely
• Be ready to pull the plug quickly if issues arise

10. Monitoring & Maintaining Inbox Deliverability

Deliverability isn't something you set up once and forget about. It requires ongoing monitoring.

What to monitor

Bounce rates
Track per domain and per mailbox. Rising bounces indicate list quality issues or infrastructure problems.

Reply rates
Sudden drops may indicate deliverability issues, not just copy problems. If your copy hasn't changed but replies have dropped, it's worth investigating.

Spam complaints
Most sequencers don't show these. If you have access to Google Postmaster Tools, it's worth keeping an eye on it.

Warmup health
Check your warmup tool's dashboard for inbox placement rates.

Warning signs

• Bounce rate climbing above 2%
• Reply rates dropping without any copy changes
• Warmup inbox placement dropping below 80%
• Increase in “email not delivered” notifications

Recovery steps

If a domain shows problems:

1. Pause all sending from that domain
2. Increase warmup volume
3. Wait 2 to 4 weeks
4. Test with small volume
5. If it's still problematic, retire the domain and rotate in a new one

Sometimes domains just get burned and there's not much you can do about it. This is why having backup domains ready is so important.

11. Common Deliverability Mistakes

Mistake 1: Sending from your primary domain

This is the big one. Never risk your main business email on cold outreach. The potential downside is just too severe.

Mistake 2: Skipping warmup

New mailboxes need 3 to 4 weeks of warmup before sending cold email. I know it's tempting to skip this step, but there really aren't any shortcuts here.

Mistake 3: Sending too many emails per mailbox

Stay at 10 cold emails per day per mailbox. Scale with more mailboxes, not more volume per mailbox.

Mistake 4: Not verifying your list

Every list needs verification, even “pre-verified” ones. Bounce rates above 2% will hurt you.

Mistake 5: Tracking open rates

Open rate tracking inserts a tiny pixel into emails. This triggers “Spam Warning” notifications in Gmail and Outlook. I'd recommend turning it off entirely.

Mistake 6: Using spammy words

Words like “free,” “guaranteed,” “act now,” and “limited time” can trigger spam filters. Write like you're emailing a colleague, not like a marketer.

Mistake 7: Including too many links

One link per email is ideal. Multiple links look promotional and can trigger filters.

Mistake 8: Using URL shorteners

Services like bit.ly and tinyurl are heavily associated with spam. Use full URLs or no links at all.

Mistake 9: Creating identical mailboxes across domains

If you create joe@company1.com, joe@company2.com, joe@company3.com across 50 domains, it looks suspicious. Vary the names.

Mistake 10: Ignoring sending patterns

Sending 1,000 emails at exactly 9:00:00am looks automated. Spread sends throughout the day with slight randomisation.

Quick Reference: The Safe Setup

• Domains: Secondary domains only, never your primary
• Mailboxes per domain: 3 (safe) or 5 (budget-conscious)
• Emails per mailbox per day: 10 cold plus 10 warmup equals 20 total
• Warmup period: 3 to 4 weeks before cold sending
• Verification: Every list, before uploading to sequencer
• Bounce rate target: Under 2%
• Open tracking: Off
• Links per email: 1 maximum

What's Next?

1. Set up your secondary domains
2. Buy mailboxes from a reputable provider
3. Start warmup (3 to 4 weeks)
4. Verify your email list
5. Start sending at low volume
6. Monitor and maintain

Deliverability is the foundation of cold email. Get this right and everything else becomes easier.

Good luck.