The DB or DBM version of the database may be rebuilt explicitly by executing the command
If the RebuildAliases (old D) option is specified in the configuration, sendmail will rebuild the alias database automatically if possible when it is out of date. Auto-rebuild can be dangerous on heavily loaded machines with large alias files; if it might take more than the rebuild timeout (option AliasWait, old a, which is normally five minutes) to rebuild the database, there is a chance that several processes will start the rebuild process simultaneously.
If you have multiple aliases databases specified, the -bi flag rebuilds all the database types it understands (for example, it can rebuild NDBM databases but not NIS databases).