Helix migrated 5M-subscriber newsletter overnight

Zero downtime. Open rate stayed within 0.2 points of baseline through cutover week.

Challenge

Helix runs a 5.2M-subscriber daily newsletter. The previous platform announced a 2x pricing increase. Switching at scale risked breaking inbox placement at exactly the wrong moment in their funding cycle.

Solution

Dedicated migration architect ran the full playbook: parallel-traffic ramp from 1% to 100% over six days, with explicit metric gates at each step. Helix kept the previous vendor warm as a rollback target.

Outcome

Zero downtime. Open rate moved by 0.2 points during cutover and recovered fully within 48 hours. Helix saved 45% on annual email cost.