The Email Deliverability Checklist (Before You Hit Send)

Most deliverability problems are preventable. The frustrating part is that senders usually discover an issue after the campaign has already gone out — bounces spike, open rates crater, or a postmaster tool quietly flags the domain as a spam source. Running through a structured checklist before you send catches the majority of these problems before they cost you.

This checklist is organised roughly in order of importance. The authentication section matters most; skip it and nothing else you do will fully compensate.

1. Authentication: SPF, DKIM, and DMARC

Inbox providers use these three DNS-based records to decide whether your mail is legitimate. All three need to be in place and correctly configured.

SPF

Your SPF record lists the servers authorised to send mail for your domain. Before you send, confirm:

• Your sending IP or mail service is included in the record.
• The record ends with ~all (softfail) or -all (hardfail) — never +all.
• You have fewer than 10 DNS lookups. Exceeding this limit causes SPF to permerror, which many providers treat as a failure.
• You have only one SPF record on the domain. Multiple TXT records with "v=spf1" will break SPF entirely.

DKIM

DKIM adds a cryptographic signature to your outgoing mail. Receivers verify it against a public key published in your DNS.

• Use a key length of at least 2048 bits. 1024-bit keys are increasingly rejected.
• Confirm the selector and domain in the DKIM signature match what is published in DNS.
• If you use a shared sending platform, make sure you have set up DKIM on your own domain, not just the platform's default signing domain.

DMARC

DMARC ties SPF and DKIM together and tells receiving servers what to do when a message fails. It also unlocks reporting so you can see who is sending mail in your name.

• Start with p=none to collect data without affecting delivery, then graduate to p=quarantine or p=reject once you are confident all legitimate sending sources are covered.
• Add an rua tag pointing to an address that will actually receive and process aggregate reports.
• Ensure at least one of SPF or DKIM passes and aligns with your From domain — DMARC requires alignment, not just a pass.

If you are unsure about your current authentication setup, run your domain through the free deliverability checker to get a clear picture before you send anything.

2. Sending Infrastructure

IP reputation and warm-up

New IP addresses have no reputation. Sending large volumes immediately from a cold IP is one of the fastest ways to get throttled or blocked. A proper warm-up means gradually increasing volume over several weeks, starting with your most engaged recipients and expanding from there. The exact ramp schedule depends on your list size and sending frequency, but the principle is always the same: earn reputation incrementally.

Dedicated IPs give you full control over your reputation, but they require consistent volume to maintain. Shared IPs spread reputation across many senders — useful for lower volumes, but you are partially dependent on others' behaviour.

Sending domain and subdomain strategy

Use a subdomain for bulk or transactional mail (for example, mail.yourdomain.com) rather than your root domain. This isolates your marketing or transactional sending from your corporate email reputation. If a campaign performs poorly, the damage is contained.

Make sure your sending domain has a valid MX record and that reverse DNS (PTR record) for your sending IP resolves back to your domain or hostname. Many providers check this.

3. List Quality

Authentication and infrastructure get your mail to the door. List quality determines whether it is welcomed inside.

• Only send to people who explicitly opted in. Purchased lists, scraped addresses, and old unengaged contacts are the primary drivers of spam complaints and hard bounces.
• Remove hard bounces immediately. Continuing to send to invalid addresses signals poor list hygiene to inbox providers.
• Suppress unsubscribes without delay. Sending to someone who has unsubscribed is both a deliverability risk and, in many jurisdictions, a legal violation.
• Monitor engagement over time. Repeatedly mailing recipients who never open, click, or interact trains inbox providers to deprioritise your mail. Sunset inactive subscribers after a reasonable window — typically 90 to 180 days depending on your send frequency.
• Use a double opt-in process where possible. It reduces fake signups and spam traps entering your list.

4. Content and Sending Behaviour

Spam filters have become sophisticated enough that no single word or phrase will automatically doom a campaign. However, certain patterns do raise risk:

• A very high image-to-text ratio, or a single large image with no text, is a common trait of spam.
• Links that redirect through multiple domains before reaching the destination can trigger filters. Use clean, direct URLs.
• Make sure your unsubscribe link is prominent and functional. Gmail and Yahoo now require one-click unsubscribe for bulk senders.
• Your From name and From address should be consistent and recognisable. Sudden changes in sender identity can hurt engagement and trigger suspicion.

5. Pre-Send Testing

Before the campaign goes out to your full list, send a test to a seed list or inbox placement tool. Check how the message renders across major clients (Gmail, Outlook, Apple Mail) and whether it lands in the inbox or spam folder. Also confirm that all links work, all images load, and the unsubscribe path functions end-to-end.

A Note on Senders in Difficult Situations

Some industries face structural deliverability challenges — not because of bad practices, but because mainstream providers apply blanket restrictions to certain categories of sender. Services like Rainmail are built specifically to support senders in those situations, providing proper authentication setup, IP warm-up management, and the infrastructure to build a legitimate sending reputation from the ground up.

Whatever your situation, the fundamentals above apply universally. Authentication, clean lists, and sending to people who want your mail are the foundation everything else is built on.