Update MySQL query in postfix setting `virtual_mailbox_domains` to query alias
domains.
```
# Part of file: /etc/postfix/mysql/virtual_mailbox_domains.cf
# Original line:
#query = SELECT domain FROM domain WHERE domain='%s' AND backupmx=0 AND active=1
# Modified:
query = SELECT domain FROM domain WHERE domain='%s' AND backupmx=0 AND active=1 UNION SELECT alias_domain.alias_domain FROM alias_domain,domain WHERE alias_domain.alias_domain='%s' AND alias_domain.active=1 AND alias_domain.target_domain=domain.domain AND domain.active=1 AND domain.backupmx=0