Email deliverability for outbound: domains, SPF, DKIM, DMARC and warm-up explained

What email deliverability means for outbound teams

Email deliverability is the share of your emails that land in the recipient's inbox rather than in spam, a promotions tab or nowhere at all. It is different from delivery rate, which only tells you the receiving server accepted the message. An email can be "delivered" and still never be seen.

For outbound teams this matters more than for marketing teams. Cold emails go to people who have never heard of you, so mailbox providers such as Gmail and Microsoft Outlook watch them closely. A campaign with a great list and a sharp message can still produce almost no replies if half of it is filtered. When we diagnose a weak campaign, deliverability is the first thing we check, before the copy, as described in our guide to raising cold email reply rates.

Deliverability rests on two layers. The technical layer proves you are who you say you are: domains and DNS records. The reputation layer shows whether recipients want your mail: bounces, complaints, replies and engagement. You need both. Perfect authentication will not save a list of people who mark you as spam.

Step 1: Use a separate sending domain

Send cold outbound from a secondary domain, not from the domain your company uses for customers, invoices and support. If the outbound domain picks up a poor reputation, your day-to-day email keeps working.

• Choose a close variant that people recognize, for example getacme.com or acmehq.com for a company at acme.com. Avoid hyphens and odd top-level domains that look disposable.
• Redirect the domain to your main website so anyone who checks it lands somewhere real.
• Create a few real inboxes under named people, such as anna@getacme.com, with a profile photo and a signature. Avoid generic addresses like sales@ for first touches.
• Spread volume across several inboxes and, as you grow, several domains, instead of pushing one inbox hard.

Buy the domain early. A domain registered yesterday and sending hundreds of emails today looks exactly like a spammer, so budget a few weeks between purchase and the first campaign.

Step 2: Set up SPF, DKIM and DMARC

SPF, DKIM and DMARC are three DNS records that prove your emails are authorized and unaltered, and tell receiving servers what to do when a message fails those checks. Since February 2024, Google requires all three from anyone sending more than 5,000 messages a day to Gmail accounts, and at least SPF or DKIM from everyone else; Yahoo introduced similar rules for bulk senders at the same time. Microsoft announced similar rules for Outlook.com in 2025. Treat them as mandatory regardless of your volume.

Start DMARC at p=none with a reporting address, read the reports for a couple of weeks, then move to quarantine once you are sure every legitimate sender passes. If you send through a sequencing tool such as Instantly, Apollo or HubSpot, follow its instructions for adding the tool to SPF and DKIM; otherwise its messages will fail authentication even though your mailbox passes.

Two more technical items are worth checking. If you track clicks, set up a custom tracking domain instead of the shared one your tool provides, because shared tracking domains carry other senders' reputation. And make sure your mailbox provider (Google Workspace or Microsoft 365) handles TLS and reverse DNS, which they do by default.

Step 3: Warm up new inboxes before campaigns

Warm-up is the period in which a new inbox sends a small, slowly growing number of emails so mailbox providers can learn that it behaves like a real person. Plan two to four weeks before a new inbox sends any campaign.

Start with a handful of emails a day and increase gradually. Many teams use automated warm-up networks built into their sending tools, which exchange messages between participating inboxes and generate opens and replies. These help, but real conversations help more: use the new inbox for genuine emails to colleagues, partners and existing contacts during the first weeks.

When campaigns start, keep warm-up running in the background at a lower level and ramp campaign volume slowly. As a working practice we keep cold volume per inbox modest, in the low dozens of new contacts per day, and add inboxes rather than raising the cap. There is no official safe number; the right limit is the one at which your placement and complaint rates stay healthy.

Step 4: Protect reputation with list quality and content

Once authentication is in place, reputation is decided by how recipients react: bounces, spam complaints, replies and deletions without reading. These are mostly list and relevance problems, not technical ones.

• Verify every address before sending and remove invalid and risky ones. A hard bounce rate above about 2% is a warning sign worth acting on.
• Target narrowly. People who match your ideal customer profile and have a reason to care are far less likely to complain. Our guide to building a usable ICP covers the filters.
• Write like a person. Plain text, short, no attachments, at most one link and no image-heavy templates in the first touch. The structure in how to write a cold email that gets replies is built for this.
• Make opting out easy. Honor every opt-out quickly and stop the sequence when someone replies, positively or not. Google asks senders to keep the spam rate reported in Postmaster Tools below 0.3% and recommends staying under 0.1%.

Outreach rules differ by country, for example CAN-SPAM in the US and GDPR in the EU, so check what applies to your market and keep an opt-out in every sequence.

How to monitor email deliverability every week

Monitor deliverability weekly with a few signals: bounce rate, spam complaints, inbox placement tests and the reply rate per inbox. A sudden drop in replies from one inbox, with the same list and copy as others, usually points to a placement problem on that inbox.

• Google Postmaster Tools shows domain reputation and spam rate for mail sent to Gmail users once you send enough volume.
• DMARC reports show who is sending on behalf of your domain and whether they pass.
• Seed or placement tests send to test inboxes at different providers and show inbox versus spam.
• Reply rate by inbox and domain, tracked in the CRM next to the funnel metrics in our outbound metrics guide.

When an inbox shows problems, pause campaigns from it, keep warm-up running, find the cause (a bad list segment, a link, a volume spike) and resume slowly. This routine is part of the weekly care we provide after implementing an outbound sales system, with AI agents watching the signals and a person deciding what to change.