Email Deliverability in 2025: How to Land in the Inbox, Not the Spam Folder

Why Deliverability Is the Foundation of Email Marketing ROI

A 40% open rate on 50% inbox placement means 20% of your list actually sees your emails. A 30% open rate on 95% inbox placement means 28.5% see them — more absolute opens despite a lower open rate, because the content is reaching people. Deliverability multiplies every other email metric; without it, the best copy in the world earns no revenue.

Gmail, Outlook, and Yahoo's filtering algorithms have become significantly more sophisticated since 2023. They evaluate not just content signals but sender reputation (domain and IP), engagement patterns, and authentication compliance. Getting deliverability right requires action on all three.

DNS Authentication: SPF, DKIM, and DMARC

SPF (Sender Policy Framework): A DNS TXT record that specifies which mail servers are authorised to send email on behalf of your domain. Without SPF, receiving servers can't verify your emails are legitimate — dramatically increasing spam classification rates.

Setting up SPF: Add a TXT record to your domain DNS: v=spf1 include:klaviyo.com include:sendgrid.net ~all (replace with your actual sending services). Check current status at MXToolbox SPF checker.

DKIM (DomainKeys Identified Mail): Adds a cryptographic signature to every email that receiving servers can verify wasn't tampered with in transit. Klaviyo and most ESPs provide DKIM keys — add the provided CNAME or TXT records to your DNS. Klaviyo's setup wizard in Account → Settings → Email Authentication guides through this.

DMARC (Domain-based Message Authentication): Builds on SPF and DKIM to tell receiving servers what to do with emails that fail authentication — nothing (p=none), quarantine (p=quarantine), or reject (p=reject). Start at p=none for monitoring, move to p=quarantine once authentication is confirmed working, then p=reject for maximum protection.

List Hygiene: The Most Under-Maintained Deliverability Factor

Sending to unengaged contacts is the fastest way to damage deliverability. ISPs track engagement patterns — if a high percentage of your emails are ignored, unsubscribed from, or marked as spam by Gmail users, Gmail reduces inbox placement for your entire domain.

List hygiene actions:

• Suppress hard bounces immediately: Klaviyo does this automatically. Any email that hard bounces (address doesn't exist) should never be retried.
• Suppress soft bounces after 3 attempts: Soft bounces (mailbox full, temporary issue) may resolve — but after 3 consecutive soft bounces, suppress the address.
• Sunset unengaged subscribers (90–180 days no open/click): Run the sunset flow (see Klaviyo Flows guide). Suppress non-responders. Your engaged list will have significantly better metrics.
• Remove role-based addresses: info@, admin@, support@ — these are often distribution lists or spam traps. Remove any that were manually added.
• Never buy email lists: Purchased lists are the single fastest way to get your domain blacklisted. Every address is an unknown — many will be spam traps.

Engagement Rate Management

ISPs — especially Gmail — use engagement signals (opens, clicks, moves to primary inbox, replies) to determine how to treat future emails from your domain. Maintaining high engagement rates is the strongest long-term deliverability signal.

Improving engagement rates:

• Segment before sending: Campaigns sent to your most engaged subscribers first (15% of list) warm up the send. If engagement is strong, gradually expand. Never blast your full list with cold campaigns.
• Send only relevant content: Irrelevant content is deleted without opening — a negative engagement signal. A smaller, highly targeted send with 40% open rate beats a full-list blast with 15% open rate for deliverability.
• Send frequency matching: Don't drop from 4 sends/week to 0 sends/month and then suddenly blast. Consistent cadence maintains sender reputation; irregular spikes trigger spam filters.
• Re-engage or remove: The sunset flow described above. Unengaged contacts who aren't sunset are a continuous drag on deliverability.

Warm-Up Protocol for New Sending Domains

A new sending domain has no reputation — ISPs have no data on whether you're a trustworthy sender or a spammer. Sending high volume from a new domain immediately will result in spam filtering for most of the sends. The warm-up protocol builds reputation gradually.

Week-by-week warm-up schedule:

• Week 1: 200 emails/day to your most engaged subscribers (recent purchasers, active openers)
• Week 2: 500/day
• Week 3: 1,000/day
• Week 4: 2,000/day
• Week 5: 5,000/day
• Week 6+: Double weekly until full volume

Only move to the next tier if the previous week achieved 20%+ open rates and under 0.1% spam complaints. If metrics fall below these thresholds, stay at current volume and investigate before scaling.