Fixing an Email Deliverability Crisis Nobody Noticed Until Open Rates Cratered

Open rates dropped from a healthy 28% to under 9% over about six weeks. Nobody flagged it as urgent until it was already a real problem, because each individual week's drop looked small enough to shrug off, and by the time the trend was undeniable, the sending domain's reputation had already taken damage that would take months to repair.

What caused it

A list import from a lead-gen partner had added about 40,000 contacts with no engagement history, and the sending platform's default behavior was to include them in the next scheduled campaign at full volume rather than warming them in gradually. A large share of that list bounced or never opened, and the major mailbox providers responded exactly as designed — they started routing more of the domain's mail to spam, for everyone, not just for the new list.

Deliverability damage doesn't announce itself. It shows up as a slow decline in open rate that looks, for the first few weeks, like nothing more than normal fluctuation.

The recovery process

• Immediately stopped sending to the unengaged imported segment entirely, rather than trying to re-engage them while the domain was already flagged.
• Set up a dedicated subdomain for any future list imports or re-engagement campaigns, isolating sender reputation risk from the primary sending domain.
• Ran a win-back campaign only to the existing engaged list, deliberately avoiding anything that might further damage reputation while it recovered.
• Monitored inbox placement directly via seed-list testing rather than relying on open rate alone, since open rate itself is distorted once deliverability is already compromised.

Recovery took about ten weeks of disciplined sending before open rates returned to baseline. The actual fix — stop sending to a bad list, isolate future risk on a subdomain — took one afternoon to implement. The expensive part was the six weeks nobody had treated a slow decline as urgent, which is exactly how deliverability problems tend to get missed until they're already costly.