Cold email deliverability: the complete guide

Deliverability is whether your email reaches the inbox. For cold email it's the whole game: a perfect message in the spam folder gets the same reply rate as no message at all, and you rarely find out it happened.

Mailbox providers decide where an email goes from four things, roughly in this order of how often they cause trouble: authentication (can they tell the email really came from your domain?), volume (is this mailbox sending like a person or like a script?), list quality (do you know the people you're writing to?) and reactions (do recipients reply, ignore, or report you?). This guide takes them one at a time, with what to check and what to do, and links to the detailed guides for each.

1. Authenticate your domain

Three DNS records prove an email came from your domain: SPF lists the servers allowed to send for it, DKIM signs each message, and DMARC tells receivers what to do when the other two fail. Google and Yahoo have required them since 2024 — SPF or DKIM from everyone, all three from bulk senders — and Microsoft has enforced similar rules for high-volume senders to Outlook.com since 2025.

What to do:

• Publish exactly one SPF record, ending in ~all or -all, with fewer than 10 DNS lookups. The SPF generator builds one.
• Turn on DKIM signing in your mailbox provider and publish its key — see the setup guides for Google Workspace, Microsoft 365 and Zoho Mail.
• Add a DMARC record, starting with p=none and a report address, then tighten. The DMARC generator writes it.
• Check the result with the free DNS checker; every problem it can find is explained on its own page.

The full explanation, including alignment and the common mistakes, is in SPF, DKIM and DMARC for cold email.

2. Send at a pace inboxes trust

Volume is judged per mailbox and over time. A mailbox that sends 10 emails a day for months and then 200 overnight looks compromised; a new mailbox that starts at 200 has no history to excuse it.

What to do:

• Ramp new mailboxes up. Start at 5–10 cold emails a day and grow over about a month. Details in ramping up a new mailbox.
• Cap each mailbox at roughly 30–50 cold emails a day once established, with an hourly cap and a gap between sends. The providers' published limits — 2,000 a day for Google Workspace — are ceilings, not targets.
• Count every campaign. Limits must cover all the campaigns a mailbox is in, follow-ups included.
• Add mailboxes to grow, rather than pushing one harder. Several mailboxes spread the risk: if one has a bad week, the others carry on.

Connect mailboxes with their provider's own sign-in where you can — Google or Microsoft — or with SMTP and IMAP for anything else; IMAP is what lets replies stop a sequence.

3. Keep bounces low

Bounces are the clearest sign that you don't know who you're emailing. Under 2% is healthy; above 5%, stop and clean the list. A single bad import can wreck a mailbox's reputation in an afternoon.

What to do:

• Check addresses before the first email: the domain must accept mail, and typos like gmial.com and throwaway inboxes should never be sent to.
• Re-check data that's more than about a month old.
• Pause a campaign the moment its bounce rate jumps, find the source, and remove it.
• Never email an address that has hard-bounced again.

How to read bounce codes, and what to do when the rate climbs, is in what's a safe bounce rate for cold email.

4. Earn replies, not complaints

Providers watch how recipients react. Replies are a strong positive signal; "Report spam" is the strongest negative one. Google asks senders to keep their reported spam rate below 0.3%, and ideally below 0.1% — for cold email that's a handful of complaints per thousand sends.

What to do:

• Write to people whose role makes your email relevant, and say why you chose them.
• Keep emails short and plain. Every link, image and tracking pixel makes an email look more like bulk mail; start with open and click tracking off.
• Follow up a few times, in the same thread, then stop — see writing cold email follow-ups that get replies.
• Make leaving easy and honour it everywhere: a line inviting a reply, an unsubscribe link and the one-click unsubscribe header. More in unsubscribes in cold email.

5. Watch the signals

Deliverability problems are quiet. Nothing tells you your emails went to spam; replies just stop. Watch for:

• A failing DNS check after someone changes a record or adds a tool.
• Rising bounces on a particular list or campaign.
• Replies dropping to zero on a campaign that used to get them.
• Errors from the provider: rate limits, authentication failures, a mailbox signed out.

When one appears, slow down rather than stop: reduce the mailbox's daily limit, fix the cause, then build back up.

A one-page checklist

1. SPF, DKIM and DMARC pass for every sending domain, and MX exists.
2. New mailboxes start at 5–10 a day and ramp up over a month.
3. Each mailbox stays at 30–50 a day across all its campaigns, with gaps between sends.
4. Every address is checked before it's emailed; bounces stay under 2%.
5. Emails are short, plain and relevant; tracking is off unless you need it.
6. Follow-ups stay in the thread and stop on a reply or an opt-out.
7. Unsubscribes and bounces are suppressed everywhere, for good.

How Outsquid helps

Outsquid was built around this list. It checks SPF, DKIM, DMARC and MX for every sending domain every hour and gives each mailbox a health score; paces every mailbox with daily and hourly limits and a gap between sends, counted across campaigns; ramps new mailboxes up on their own; checks addresses before sending; and pauses a campaign or a mailbox when bounces climb. See deliverability and safeguards.