MX Pattern Grouping verifiedimportantWritten by the Hub
Treats every domain whose mail is handled by the same servers as one destination for throttling.
In detail
Domains are grouped by the shape of their MX records, so that a limit set on the group covers every domain behind it — including the corporate domains hosted by a large provider, which is exactly the case a per-domain limit misses. The same screen carries Organizational Domains, a shorter road to the same idea: match on the organisational part of the MX name instead of requiring the whole record to match. For Hub this is the difference between a limit that means "Gmail" and a limit that means "gmail.com" — the first is what an operator intends, and only this screen provides it.