When a user changes their email address, it will be updated on the wiki where they did the address change, and in the global database. It won't be updated in other wikis in the same CentralAuth cluster, however, until `AuthPlugin::updateUser` is called on that specific wiki. IIRC this only happens when the user logs in manually (via `Special:UserLogin`) on that specific wiki. This won't affect MediaWiki's operation as the email address is read from the global database anyway, but it means that old email addresses can persist indefinitely in the database. This arguably violates the [[ http://oecdprivacy.org/#quality | data quality principle ]], a fundamental rule of EU privacy regulation, and is in general bad form - we should not store private data that we don't need. It can also lead to mistakes when people look for email addresses at the wrong places.