Definition
Email deliverability is whether your email actually arrives in the recipient's inbox. It covers the technical signals that prove who you say you are (SPF, DKIM, DMARC), the reputation your sending domain and IPs have built with mailbox providers like Gmail and Outlook, the quality of your recipient list, and the operational practices that keep all of it healthy over time. Most people mistake it for spam-folder placement. It is actually a continuous score, not a binary verdict.
Why people get it wrong
The most common confusion is between "delivery" and "deliverability." Delivery is what your ESP reports to you: the recipient's server accepted the message. Deliverability is what happens after that handoff. Inbox placement, folder placement, even silent suppression by the mailbox provider — these are deliverability outcomes, not delivery outcomes. An ESP can show you 99.9% delivery while half your messages land in spam. The numbers do not contradict each other; they answer different questions.
The second confusion is treating it as a one-time project. "We set up DMARC, we are done." DMARC is one signal among many, and ISP weighting changes every quarter. Gmail's 2024 enforcement update shifted how engagement signals factor in. Outlook tightened complaint thresholds. Yahoo started caring about list-unsubscribe header presence in ways that broke some senders overnight. Deliverability is something you maintain, like database health, not something you ship.
The five things that decide your deliverability
Authentication. Your domain has SPF, DKIM, and DMARC records in DNS, and they all align. Without these, mailbox providers cannot prove you are who you claim, and bulk-sender policies will quietly route you to spam. Authentication is table stakes. Run our deliverability check if you are not sure where you stand.
Reputation. Mailbox providers track a quiet score for every sending IP and every sending domain. The score is based on bounce rate, complaint rate, spam-trap hits, engagement signals (opens, clicks, replies, marks-as-not-spam), and the consistency of your sending volume. Reputation is sticky. Building it back after a bad incident takes weeks. The fastest way to lose it is to send to old, unengaged lists.
List hygiene. The recipients on your list need to actually want what you are sending. Old contacts go cold. Bought lists are radioactive. Anyone who has not engaged in 90 days is statistically more likely to mark you as spam than to open the message. Pruning is uncomfortable because it makes your list smaller, but smaller lists outperform big stale ones on every metric that matters.
ISP-specific behaviour. Gmail does not weigh signals the same way Outlook does. Yahoo's enforcement tightened in 2024. Comcast and Verizon-class consumer ISPs have their own quirks. Granular monitoring matters more than aggregate metrics — a 2% Gmail bounce rate is a fire alarm even if your overall bounce rate looks fine.
Operations. Domain warmup when you start sending. Throttling when ISP signals get rough. Trickle recovery after a reputation incident. Isolating transactional reputation from marketing reputation so a bad campaign does not torch your password resets. This is the operational layer that turns the first four into a working system. Tools like our reputation management layer handle most of it automatically, but you can do it manually if you have the team for it.
When deliverability becomes existential
For a B2B SaaS, the transactional rail is usually the place. Password resets, OTPs, invoice notifications, customer support replies. If those land in spam, your product breaks. Customers cannot log in. Payments do not get receipted. Support emails get lost. The first time a deliverability incident hits transactional, the founder usually realises the entire company depends on this thing they have never thought about.
Marketing email is more forgiving because the cost of a missed send is lower. But marketing is where most deliverability damage starts. Bad list, aggressive cadence, weak engagement — the marketing rail starts dragging reputation down, and if the transactional and marketing share a sending domain, the damage bleeds across. This is why we ship per-domain reputation isolation as a default rather than an upgrade. The multi-level email reputation piece on our blog goes deeper if you want the architectural version.
Related reading
- The Email Deliverability Guide — full-length practical guide covering authentication setup, monitoring, and incident response.
- Domain temperature and email warmup — why warmup matters and how Cortex handles it automatically.
- Adaptive rate limiting in email — how ISP feedback should control your sending speed in real time.
- Run a free deliverability check on your domain — instant audit of your SPF, DKIM, DMARC, and MX records.