Cold Email Deliverability: 12 Rules for B2B Senders in 2026

A cold email campaign is only as effective as its inbox placement. You can write the best subject line in the world, but if it lands in the spam folder, no one will ever read it. Deliverability decides whether any of the rest of the work is seen.

Mailbox providers have tightened what they expect from senders. Google's email sender guidelines, which have applied to anyone sending to Gmail accounts since 1 February 2024, set out authentication and spam-rate requirements worth treating as the baseline. This guide walks through the 12 rules we would ask any B2B sender to follow.

1. Authenticate your sending domain

Google's sender guidelines require everyone sending to Gmail accounts to set up SPF or DKIM, and anyone sending 5,000 or more messages a day to set up DMARC as well, with the From domain aligned to the SPF or DKIM domain. Configure all three before sending a single cold email, and check them with a tool such as MXToolbox or dmarcian.

• SPF: authorizes your sending servers
• DKIM: cryptographically signs each email
• DMARC: tells inboxes what to do if authentication fails (start with p=none, move to p=quarantine once clean)

2. Use a separate sending domain

Never send cold email from your main company domain. If deliverability tanks, it can take your sales reps, marketing newsletters, and billing notifications down with it. Purchase a lookalike domain (yourcompany.co instead of yourcompany.com) and route all outbound campaigns through it.

3. Warm up new domains and mailboxes

A brand-new mailbox blasting 500 emails a day looks like a spam bot to every filter. Warm up each mailbox over 3 to 6 weeks by starting at 20 to 30 emails per day and gradually increasing volume. Use a reputable warmup service that simulates natural engagement (opens, replies, folder moves).

4. Keep daily volume per mailbox below 50

We keep each mailbox under 50 cold sends a day and scale by adding mailboxes rather than pushing more volume through one. Sending harder from a single mailbox is what invites throttling, which is why high-volume programmes rotate through a set of warmed mailboxes on every campaign.

5. Verify every email before sending

Bounces damage deliverability, because each one tells the receiving provider that your list is out of date. Keep them as close to zero as you can. Use a multi-source verification tool that catches catch-alls, role-based addresses, and temporary mailboxes. Re-verify lists that are more than 30 days old before sending.

6. Avoid spam trigger words and phrases

Modern filters use machine learning, not just keyword matching, but old-school red flags still raise scores. Skip phrases like 'act now', 'guaranteed', 'no obligation', and 'special promotion'. Avoid ALL CAPS, excessive exclamation points, and emoji spam.

7. Write short, personal, human emails

How recipients respond matters. Google's sender guidelines ask senders to keep the spam rate reported in Postmaster Tools below 0.1% and never to reach 0.3%, and the practical way to keep complaints low is to send email people are glad to reply to. Short emails, roughly 50 to 125 words, read like a message from a person rather than a campaign, and they are far easier to answer from a phone. Personalise at least one sentence with research about the prospect.

8. Plain text beats HTML for cold

Heavy HTML templates with images and tracking pixels scream 'marketing email' to spam filters. For cold outreach, send plain text with minimal formatting. If you must use HTML, keep it simple and match how a real person would write from their regular inbox.

9. Remove tracking pixels on first touch

Open tracking works by adding a hidden image to each message, which makes a cold email look more like bulk marketing than a note from a person, and open data is less reliable than it was because some mail apps load images automatically. Disable open tracking on your first touch, and enable it only on follow-up emails if you need it. Check whether your tool lets you switch it off per sequence.

10. Maintain a tight reply loop

Inboxes reward two-way conversations. When someone replies, respond within 4 hours during business hours. Unsubscribe negative replies immediately. A quick unsubscribe prevents spam complaints that damage your reputation.

11. Monitor sender reputation weekly

Use Google Postmaster Tools and Microsoft SNDS to monitor your sender reputation weekly. Watch for drops in domain reputation, spike in spam rates, or new blacklist entries. Address issues within 24 to 48 hours before they cascade.

12. Include a clear opt-out

Every marketing email must say who it is from and give the recipient a valid address to ask you to stop. That is regulation 23 of PECR, and it applies whether you are emailing a sole trader or a limited company. Under UK GDPR, the ICO describes the right to object to direct marketing as absolute, so a request to stop must be honoured, and the address suppressed rather than simply deleted so that it is never imported again. A plain 'reply stop if this is not relevant' line or a working unsubscribe link does the job and reduces complaints. Google's guidelines also require one-click unsubscribe on marketing messages from senders at volume. If you email people outside the UK, their local rules apply too, and some are stricter than the UK's.

Bottom line

Cold email deliverability is a compounding game, and each rule reinforces the others. What these 12 rules control is the part you own: authentication, separation, warming, list hygiene, volume discipline and reputation monitoring. Where a message finally lands is decided by the receiving provider, so nobody can promise you the primary inbox. What you can do is remove every reason a filter has to doubt you. If deliverability is killing your campaigns, the problem is almost never the copy. It is the infrastructure.