Email deliverability

A marketing agency managing cold outreach campaigns noticed increasing bounce rates and spam complaints. They discovered the team had been sending to large purchased lists without validation, and many addresses had marked emails as spam out of confusion (unsolicited contact). The agency implemented pre-send list validation, removed all addresses with no engagement in previous campaigns, and added preference centre options. They also transitioned away from purchased lists toward organic lead generation. Within 6 weeks, bounce rates dropped from 12% to 2%, complaint rates fell from 1.5% to 0.1%, and overall deliverability improved from 68% to 91%.

A B2B software company noticed declining campaign response rates over three months. They assumed the market was saturated and considered changing messaging. Then they checked mail server logs and discovered only 45% of emails were being accepted by ISPs: the rest were being silently rejected or filtered. Investigation revealed they had not implemented DKIM signing and had poor SPF configuration. Once they fixed authentication, inbox placement jumped to 82% with no message changes required. Campaign response rates recovered immediately.

Deliverability is the foundation of all email marketing effectiveness. The most brilliant campaign copy achieves nothing if emails never reach inboxes. In B2B contexts, a 50% deliverability rate means half your qualified prospect list never sees your message, directly impacting pipeline and revenue.

Deliverability problems compound silently. A campaign with 70% deliverability feels like a conversion problem (why aren't people responding?), but it's actually an infrastructure problem. Teams often optimise messaging and targeting when the real issue is technical. Fixing deliverability often reveals true message performance.

ISPs measure deliverability through engagement signals. They track open rates, click rates, and complaint rates. Low engagement tells ISPs that recipients don't want your emails, resulting in stricter filtering on future sends. Conversely, strong engagement signals improve deliverability on subsequent campaigns, creating a virtuous cycle.

Implement proper authentication infrastructure before sending campaigns. Configure SPF records to authorise specific mail servers. Implement DKIM signing to prove message authenticity. Set up DMARC policy to define how receiving systems handle authentication failures. These three protocols form the foundation; without them, deliverability will suffer.

Monitor deliverability metrics constantly. Track bounce rates by type: soft bounces (temporary failures) should clear quickly, while hard bounces (permanent failures) must be removed from lists immediately. Monitor spam complaint rates and suppress complainers to prevent list degradation. Track engagement by ISP domain to identify problem areas (Gmail, Outlook, corporate domains, etc.).

Maintain list quality aggressively. Remove invalid addresses before sending. Use list validation services to catch problematic addresses. Segment lists by engagement level and treat inactive segments differently: they create low engagement signals that harm reputation. Remove addresses that bounce or complain; keeping them damages future deliverability.

Plan sending volume carefully. New domains or IPs must warm up gradually: start with small daily volumes and increase over weeks. Sending 1000 emails daily from an IP with no history triggers ISP suspicion. Consistent volume signals legitimacy; erratic spikes signal spam.

An enterprise software company launched a new product line and created a separate sending domain for product-specific campaigns. Rather than immediately sending volumes, they implemented gradual IP warming: starting with 100 daily emails to their most engaged customers, then doubling volume every 3 days. They monitored bounce and complaint rates daily, removed any problematic addresses immediately, and maintained consistent timing. After 3 weeks, the new domain had sufficient reputation to send 10000 daily emails with 89% inbox placement, comparable to their established domain.

Email deliverability is whether your email actually lands in the inbox, not the spam folder and not silently dropped on the way. It splits into two questions: did the receiving mail server accept the message at all, and once accepted, did it survive filtering and reach the primary inbox instead of being shunted to spam or promotions.

The nasty part is that bad deliverability is invisible. Your tool happily reports the email as "sent", but the recipient never sees it. Mailbox providers like Gmail and Outlook reject or filter quietly, so you end up blaming your copy or your offer when the real problem is plumbing.

A handful of things move the needle:

• Authentication , SPF, DKIM and DMARC are DNS records that prove the email genuinely came from you. Skip them and you look like a forger.
• Reputation , providers score your sending domain and IP on history. Spam complaints and bounces drag it down; steady, wanted mail lifts it.
• List quality , sending to dead or scraped addresses bounces hard and tanks your standing fast.
• Volume consistency , a brand-new domain blasting 1,000 emails on day one screams spammer. Ramp gradually ("warming up").
• Engagement , opens and replies tell providers people want your mail, which earns you better placement next time. It compounds either way.

How to apply it

Get authentication right before you send a single campaign. Then watch your metrics: separate soft bounces (temporary, ignore) from hard bounces (dead, remove immediately), suppress anyone who marks you as spam, and prune addresses that never engage. New domain or IP? Warm it slowly , start small with your most engaged contacts and climb over a few weeks.

Examples

Say you're running cold outreach with Instantly across several mailboxes. Deliverability is the whole game here: Instantly rotates sending across inboxes and runs warm-up so each one builds reputation before you scale volume. If your DKIM and DMARC aren't set on every sending domain, even a perfect sequence quietly rots in spam , and you'd see it in plummeting reply rates, not in a bounce report.

Say you're sending a weekly newsletter through Brevo. Because you're emailing people who opted in, your job is keeping engagement high and the list clean: let Brevo auto-suppress hard bounces and unsubscribes, and quietly drop contacts who haven't opened anything in months. Those dead weights generate the low-engagement signal that pulls your whole sender reputation down.

Say you've moved your lifecycle emails into Customer.io and you're spinning up a fresh sending subdomain for them. Don't open the taps , warm it up: start with a few hundred sends a day to your most active users, double every few days, and watch bounce and complaint rates daily. After a few weeks the new subdomain has earned the reputation to send at full volume without landing everyone in spam.