The complete guide to email deliverability

What deliverability is, and how it differs from delivery

Delivery means the receiving mail server accepted your message and did not bounce it. Deliverability means that accepted message actually landed in the inbox, rather than in spam, junk or a promotions tab nobody checks. The gap between the two is where revenue is won or lost.

ESPs report delivery rates close to 99% for almost everyone, which is why the number is essentially useless on its own. The metric that matters is inbox placement: of the messages that were delivered, how many a human will ever see.

The four things that decide deliverability

Mailbox providers like Gmail, Outlook and Yahoo make placement decisions on every message. The decision is opaque, but the inputs are not. Four categories drive it:

1. Authentication. SPF, DKIM and DMARC prove that you really are who you claim to be.
2. Reputation. The track record of your domain and IPs, judged on engagement, complaints, bounces and spam-trap hits.
3. List quality. Whether the addresses you send to are real, opted-in and engaged.
4. Content and engagement. Whether the message itself, and the way people respond to it, looks like wanted mail.

Weakness in any one of the four pulls the others down. A perfectly authenticated send to a dirty list still tanks reputation. A clean list sent without DMARC alignment still gets spoofed and filtered.

Authentication: SPF, DKIM and DMARC

SPF lists which servers may send mail for your domain. DKIM signs every message with a cryptographic key so receivers can verify it was not tampered with. DMARC sits on top of both and tells receivers what to do when authentication fails, and where to send reports.

From 2024 onwards Gmail and Yahoo require SPF, DKIM and a DMARC policy for any sender pushing more than 5,000 messages a day. If you do not have all three, you do not get a fair hearing. We cover the records and setup in the dedicated guide below.

Domain and IP reputation

Reputation has two layers. Domain reputation follows your sending domain across every inbox provider and every IP it sends from. IP reputation lives on the specific sending IP. Domain reputation is now far more important than IP reputation for most senders.

Both are scored on the same signals: how often people open and click, how often they complain, how many addresses bounce and whether you have hit a spam trap. Both are rebuilt slowly and lost quickly. The job of a deliverability programme is to keep both green.

Warming up a new sending domain

A brand-new domain has no reputation. Sending full volume from a cold domain is the fastest way to get filtered. Warm-up means ramping volume slowly, starting with your most engaged contacts, so providers see strong positive signals from day one. A safe ramp takes three to four weeks. We cover the schedule and the pitfalls in the warm-up guide.

List quality and verification

Most deliverability problems are list problems wearing a deliverability mask. Verify any list before its first send and re-verify regularly. Suppress hard bounces immediately. Remove addresses that have not engaged in 90 to 180 days, depending on cadence. Honour every unsubscribe and complaint, every time.

Content, engagement and complaints

Modern spam filtering is overwhelmingly behavioural. Subject-line word lists matter very little. What matters is whether the people you send to open, click, reply or hit the spam button. Send wanted content to engaged people and content rarely becomes the problem.

Two specifics still help. Include a plain-text version alongside the HTML, and include the List-Unsubscribe header so recipients can opt out from the inbox without ever hitting the spam button.

Sending at scale across multiple domains

Past roughly 500,000 recipients per send, one domain stops being safe. Mailbox providers throttle, a single bad day can torch the whole programme, and you have nowhere to fail over. Operators sending at scale run a warmed pool of sending domains, balance load across them and isolate any reputation incident to a single member of the pool.

How to monitor and protect deliverability

Deliverability is not a setting; it is a running discipline. The non-negotiable signals to watch are:

• Bounce rate per send, with hard and soft bounces split out.
• Spam complaint rate per send and per inbox provider.
• Inbox placement rate by provider, measured against a seed list.
• DMARC aggregate reports for spoofing and alignment failures.
• Domain and IP reputation in Google Postmaster Tools and Microsoft SNDS.

When any of these moves the wrong way, throttle or pause that domain before reputation slips further. Done by hand this is exhausting; done automatically it is invisible.

When a managed service makes sense

If your list is small, sending is occasional and your team has the appetite, an ESP and good discipline will do. If your list is in the hundreds of thousands or millions, deliverability has to run itself: warmed multi-domain pools, three-provider failover, real-time reputation guardrails and a team that fixes problems before you ever see them. That is exactly what Fortitude Send does, end to end, so you approve the work and never think about the inbox.

Frequently asked questions