Email Deliverability Calculator

The underlying calculation

Deliverability is the share of sent messages that reach an inbox rather than a spam folder or a block. It is distinct from delivery rate, which only counts whether the message was accepted.

Delivery rate = (sent − bounces) ÷ sent. Inbox placement rate = inbox arrivals ÷ delivered, and the second cannot be measured from your own platform's reporting.

A campaign can show 98.5% delivery and 60% inbox placement, and nothing in the standard report distinguishes the two.

Worked through

11,820 delivered from 12,000 sent is a 98.5% delivery rate, which looks excellent.

If inbox placement is 78%, then 9,220 messages reached an inbox and 2,600 went to spam. The 2,600 still count as delivered and still appear in the denominator of every rate.

That means the reported 22% open rate is really 28% of the messages people could see, and, more importantly, 22% of the list is generating negative engagement signals that worsen placement further.

The 2,600 lost messages at $0.118 of revenue each is $307 of revenue per campaign, or nearly $16,000 a year on weekly sends.

Where it goes wrong

Standard platform reporting cannot see inbox placement, so a programme can decline for months while every visible metric looks acceptable. Seed list testing or a dedicated monitoring service is the only way to measure it.

Placement also varies by provider, a domain can have good placement at one major provider and poor placement at another, and an aggregate figure hides that entirely.

Making it useful

Authenticate properly with SPF, DKIM and DMARC. These are configuration rather than strategy, they are the first thing providers check, and a surprising number of sending domains still fail one of them.

Then manage engagement actively: suppress subscribers who have not opened or clicked in six months, and re-engage them separately. Sending to disengaged addresses is the largest controllable cause of placement decline.

Why the sender reputation loop is self-reinforcing

Providers judge a sender on how recipients behave: opens, clicks, replies, deletions without reading, and spam complaints. Good behaviour improves placement, which exposes messages to more engaged recipients, which improves behaviour further.

The loop runs the other way just as strongly. Poor placement means fewer people see the message, engagement metrics fall, and placement worsens, a decline that accelerates once it starts and is slow to reverse.

That asymmetry is why list hygiene is worth doing before there is a problem. Suppressing a disengaged third of a list costs a third of the visible reach and protects the placement of the remainder, and businesses that do it consistently rarely have deliverability crises at all.

Sending domain choice matters more than most configuration decisions: marketing mail sent from the primary domain puts transactional and business email at risk if reputation deteriorates.

A dedicated subdomain isolates the risk, and it is far easier to set up before there is a problem than to migrate to afterwards.

Warming a new sending domain gradually over several weeks avoids the volume spike that providers treat as a spam signal, and skipping it is the most common cause of a failed migration.