What are signs that migration hurt deliverability?

Monitor closely during and after migration for these warning signs of deliverability damage.

Metric warning signs:

Rising bounce rates: New IPs may face stricter filtering initially, but unusual spikes indicate problems

Complaint increases: Suppression gaps or changed sending patterns triggering complaints

Open rate drops: May indicate spam folder placement (though MPP makes this less reliable)

Click rate drops: More reliable engagement signal indicating inbox issues

Reputation indicators:

Postmaster Tools declines: Google showing reputation drop for your domain

SNDS warnings: Microsoft flagging IP reputation issues

Blocklist appearances: New IPs getting listed

Delivery pattern changes:

Deferrals increasing: Providers throttling your new infrastructure

Specific domain problems: One major provider suddenly rejecting

Delivery slowdown: Messages taking longer to deliver

How to respond:

Slow down sending volume

Shift traffic back to old ESP if needed

Review suppression list completeness

Check authentication configuration

Extend warmup period

Focus on most engaged segments

If the new ship is taking on water, return to the old harbor until repairs are complete. Don't force a failing migration.