Customer.io email deliverability trend

Workspace 199348. Every email send (campaigns, broadcasts, transactional). Pulled 1 June 2026. All open and click rates are human only, with Apple Mail Privacy and prefetch bots stripped out.

Is deliverability trending down?

No. The trend never broke. One cold send did.

Through April, deliverability was steady and improving. Bounce rate fell from 5.8 percent in December to 3.1 percent in April, and delivery held near 95 percent. May looks alarming on the monthly average, but that is almost entirely one broadcast: the Dormant winback to a cold list (ID 97), which sent 189,749 emails at a 9.0 percent bounce rate for 31 conversions. Remove that one send and May becomes your healthiest month on record: bounce 1.9 percent, delivery 97.3 percent, human open 17.7 percent. The real risk is not slow decay. It is that a cold blast like this can damage sender reputation and drag down your healthy everyday mail.

May, with and without the cold blast

Exclude the Dormant winback blast (broadcast 97). Flip this to see the underlying trend with the one cold send removed. It updates the cards, the monthly charts, the weekly chart and the table.

Showing: as sent

The long view, by month

Send volume

Emails sent versus delivered each month. The email program in this workspace begins in November 2025.

May tripled to 289k against a 60k to 95k monthly norm. That jump is a deliberate blast, not organic growth.

Human engagement

Human open and human click rate, as a share of delivered.

Open rate held in a 17 to 27 percent band, then fell to 11.5 percent in May. Exclude the winback and it stays in band.

Deliverability

Bounce rate as a share of sent, and unsubscribe rate as a share of delivered.

Bounce had improved to roughly 3 percent, then jumped to 6.6 percent in May. Without the winback it is 1.9 percent.

Weekly zoom, last 11 completed weeks

Where it broke: the week of 24 May

Volume as bars, bounce rate and human open rate as lines. Week labels use a Sunday start.

That week sent 232k against an 11k to 30k norm, bounced 7.6 percent, and saw opens drop to 9.8 percent. Every other week sits in a healthy band.

Daily pinpoint, last 44 days

The blast landed 27 to 29 May

Daily sends as bars and bounce rate as a line. Today, 1 June, is partial and is left off the rate line.

Three days of 50k, 80k and 79k sends. Bounce rate ran 7.3, 7.0 then 10.2 percent, with 661 unsubscribes on 29 May alone.

Monthly detail

What it means and what to do

This was an event, not a trend. Your baseline of triggered campaigns and targeted broadcasts is healthy and was getting better from December to April. The May monthly average is one broadcast pulling the whole month down.
The Dormant winback is a reputation risk. A 9.0 percent bounce, a 0.9 percent unsubscribe rate and an 8 percent open rate are the signature of mailing a large cold list. Gmail and Yahoo throttle a domain when bounces and complaints stay high, and that then suppresses your good mail.
Suppress the 17,110 bounces before the next big send. Confirm the "Email ever bounced" exclusion, segment 160, is negated in every broadcast audience so these addresses are never mailed again.
Do not blast cold segments at full volume. If a winback is worth running, throttle it with a warm up ramp rather than 80k in a day, and gate it on a minimum engagement window such as opened or active in the last 90 days.
Watch the next 2 to 4 weeks. If human open rate on your normal triggered campaigns dips after this blast, that is reputation contagion, and that is the real cost. I can pull a follow up in mid June.
On opens. Roughly 59 percent of May raw opens were machine, from Apple Mail Privacy and prefetch. Every rate here uses human only opens, so total open dashboards will read rosier than reality.

Method. Source is the Customer.io all_metrics endpoint for workspace 199348, which aggregates every email send across campaigns, broadcasts and transactional, so it reflects domain level deliverability rather than a single message. The winback figures come from the broadcast 97 metrics endpoint. Bounce rate is bounced divided by sent. Open, click and unsubscribe rates are divided by delivered. Human opens and clicks exclude Apple Mail Privacy and prefetch bots. Partial current periods, meaning the June month, the current week and today, are left out of rate calculations. SMS on Twilio is tracked separately and is not shown here.