Missing foreign keys are (at least for MySQL/MariaDB):

• fetchmail.destuser -> mailbox->username
• quota.username -> mailbox->username
• quota2.username -> mailbox->username
• vacation.email -> mailbox->username
• domain_admins.domain -> domains.domain
• domain_admins.username -> admins.username (I'm unsure whether this is already there)

This also requires some cleanup steps beforehand such as DELETE FROM $vaction where email not in (SELECT username from $mailbox) etc., because otherwise creating foreign keys may fail.