Switching platforms is high-risk if you do it wrong. Here is the playbook that works at any scale.
Set up parallel infrastructure. Add Inbox OSS sending domain CNAMEs alongside your existing provider. Both vendors live, no DNS conflict.
Mirror 1% of traffic. Use a feature flag in your send pipeline to route 1% to the new platform. Watch deliverability scorecards side by side for 24-48 hours.
Ramp gradually. 1% to 5% to 25% to 50% to 100% over 7-14 days. Each ramp step has explicit metric gates: open rate within 1 point, bounce rate within 0.1 point, complaint rate within 0.02 point.
Keep the old platform warm. Until you have run 100% for at least 7 days. Rollback is your insurance.
Mirror suppressions. Export from old platform, import to new. Inbox OSS accepts SendGrid and Mailgun CSV formats verbatim.
Communicate internally. Engineering, marketing, customer support all need to know what is happening. A surprise bounce-rate spike during migration looks like a fire.
Decommission carefully. Final step is removing DNS, deleting the old account, archiving any historical data you need.