mx_patterns verifiedimportantWritten by the Hub
MX patterns
Patterns matched against a domain's MX records, so domains served by the same provider fall into one group.
In detail
The mechanism that makes "limit everything behind Google's mail servers" expressible in one rule instead of thousands. For a sender whose audience includes corporate domains — which quietly use the same few providers — this is the difference between limits that work and limits that look right on paper. Undocumented as a table, though the console explains the feature well.