Deliverability

Engagement (opens, clicks, replies, forwards) is one of the top signals modern spam filters use. Low-engagement senders are routed to spam even with perfect authentication. Segmenting by engagement and removing dormant subscribers protects your overall placement.

Quarterly at minimum for active lists. Before every major campaign. After any import from a third party. Real-time verification on signup forms via the API closes the gap between cleanings.

Hard bounce = permanent (address does not exist) → remove from list immediately. Soft bounce = temporary (inbox full, server down, recipient blocked) → retry, then suppress after 3 consecutive failures. Both count against your bounce rate.

A prioritized, code-anchored checklist: authenticate properly (SPF/DKIM/DMARC), verify your list, segment by engagement, watch bounce + complaint rates, warm up new infrastructure, use real From addresses, monitor placement.

Verify before every send; segment by engagement; authenticate properly; clean opt-ins; suppress hard bounces aggressively; watch the rate weekly. Concrete steps and the order to do them in.

Adding an SPF record means publishing a single TXT entry in your domain DNS that lists every server allowed to send mail as you. Walkthrough for the common platforms (Google Workspace, Microsoft 365, SendGrid, Mailgun) plus our free SPF generator.

DKIM setup is per-ESP. Generate a key in your sending platform, publish the public key as a DNS TXT record on a selector subdomain, then enable signing. Walkthrough for the major ESPs plus our free DKIM tools.

Don't jump straight to p=reject. Start at p=none and watch the aggregate reports. Once your legitimate senders are all aligning, progress to p=quarantine, then p=reject. Full timeline + the exact records to publish at each stage.

Domain warm-up is the gradual ramp from zero to full sending volume on a fresh domain. The right curve is 4 to 8 weeks, doubling roughly every 3 to 5 days, with engagement-weighted recipients. Skip it and you'll be flagged as suspicious from day one.

Warming a fresh IP follows the same shape as domain warm-up but with even stricter volume increments because IP-level reputation is more granular. 4-6 weeks, engagement-first recipients, careful daily monitoring.

Under 2% is healthy. 2–5% is a yellow flag worth investigating. Above 5% is actively damaging your sender reputation. Cold-outreach campaigns often see 15–30% on unverified lists, which is why verification first is essential.

A suppression list is the set of addresses your ESP will never send to, regardless of which campaign or segment includes them. Hard-bounced, unsubscribed, complained, and explicitly suppressed addresses all go here. Universal across your whole account.

An email blacklist (DNSBL) is a list of sending IPs or domains that some mail providers refuse to accept mail from. Spamhaus, Barracuda, and SpamCop are the major ones. Free tools at MXToolbox and similar can check your status across multiple lists.

SPF (Sender Policy Framework) is a DNS TXT record listing every server and IP allowed to send email from your domain. Receiving mail servers check it on every inbound message. Fail SPF and your mail likely lands in spam. Free SPF checker and generator on our site.

BIMI (Brand Indicators for Message Identification) displays your logo next to authenticated emails in supporting inboxes (Gmail, Yahoo, Apple Mail). Requires DMARC at p=quarantine or p=reject + a VMC certificate. Optional but increasing brand recognition.

DKIM (DomainKeys Identified Mail) cryptographically signs every outbound message with a private key. Receiving servers verify the signature against the public key in your DNS. Tampered or spoofed messages fail the check.

DMARC ties SPF and DKIM together with an enforcement policy. It tells receiving servers what to do with mail that fails authentication (nothing, quarantine, or reject) and gives you reports on who's spoofing your domain. Gmail and Yahoo require DMARC for bulk senders as of 2024.

Greylisting is a server-side anti-spam technique that intentionally rejects the first delivery attempt from any sender, expecting legitimate senders to retry. It can show up as `unknown` results during verification. The engine retries internally and refunds the credit if it still cannot get a definitive answer.

Sender reputation is the score ISPs assign to your domain and sending IP. Built from authentication, bounce rate, complaint rate, engagement, and consistency. Verification is the foundation; everything else compounds from there.

Missing authentication, high bounce rate, low engagement, spam-trigger content, IP/domain blacklist, no list hygiene, abrupt volume changes. Each reason matched to its fix.