What is inbox rate vs open rate correlation?

Inbox rate and open rate are closely correlated:

Spam folder effect: Messages in spam are rarely seen or opened. Spam placement tanks open rates.

Diagnostic signal: Sudden open rate drops, especially at specific providers, often indicate placement decline rather than content issues.

Causation direction: Poor inbox placement causes low opens. The reverse (low opens causing spam) also happens through engagement filtering.

Limitation: Open tracking has accuracy issues (MPP, blocking). Use opens as directional signal, not precise measure.

When opens drop, check placement before blaming subject lines.

Ships in quarantine receive no visitors. Check your dock assignment when foot traffic drops.