x-mq-tag verifiedcritical

The single most useful header for an integration, because a tag is what everything afterwards selects on: statistics, the delivery log, pauses, forced errors, response patterns and rewrite rules. An exporter that tags each message with its campaign gives the operator the ability to pause or cancel that campaign on the node — from a system that did nothing except add a header. That is the whole control module's selector arriving for free.