What are signs that migration hurt deliverability?

You've migrated to a new ESP. The first week looks fine. Then, slowly, things start shifting. Open rates down a bit. Bounce rates up. Some campaigns that used to land cleanly start going to spam. Here's how to tell if your migration is the cause and what signals to watch.

The most obvious indicator is a sudden increase in bounce rates, particularly hard bounces or persistent soft bounce clusters, after you start sending from new IPs. Some rise is normal during IP warmup as providers apply stricter scrutiny to unknown senders. But a sharp jump, especially to specific mailbox providers, suggests something is wrong: a missing suppression, a warmup schedule that's moving too fast, or authentication that isn't set up correctly.

Complaint increases are another warning sign. If your unsubscribe rate or spam complaint rate climbs after migration, it often means either your suppression lists didn't transfer completely (you're emailing people who already opted out) or your sending patterns changed in a way that surprised your subscribers.

Watch Google Postmaster Tools and Microsoft SNDS during the transition. If your domain reputation in Postmaster drops from "high" to "medium" or your SNDS IP status turns yellow, that's a concrete signal that the mailbox providers are responding negatively to your new sending pattern. Both tools update frequently enough to catch problems within a few days of when they start. Authentication failures are also a common migration casualty. Verify your SPF and DKIM with our free tools and confirm all three authentication protocols pass before you conclude the issue is something else. Misconfigured authentication is the first thing to rule out.