Routing rules in the Admin Console let you send mail to more than one destination, apply conditions to where mail goes, or forward mail matching certain criteria. They are powerful, and also one of the easier things to misconfigure in a way that is not obvious until mail starts disappearing.
Split delivery
Split delivery sends a copy of incoming mail to both Google Workspace and another mail server — often used temporarily during a migration. Left in place after the migration is actually finished, it causes duplicate mail delivery indefinitely, which is a common leftover configuration we find on older accounts.
Catch-all addresses
A catch-all rule captures mail sent to any address at your domain, including typos and addresses that do not officially exist. Without careful scoping, a catch-all can also capture and misroute mail intended for addresses you deliberately do not want to receive mail at.
Rule order and loops
Routing rules are evaluated in order, and a rule that forwards mail back into a condition matched by an earlier rule can create a routing loop. These are usually the hardest routing problems to diagnose because the symptom — mail simply vanishing or bouncing with a generic error — does not obviously point back to the rule causing it.