Receive mail for multiple domains

Add to the end of your /volume1/@appstore/MailServer/etc/main.cf:

virtual_alias_domains = seconddomain.tld thirddomain.tld
virtual_alias_maps = hash:/volume1/@appstore/MailServer/etc/virtual

Create the file //volume1/@appstore/MailServer/etc/virtual and use this format:

address1@seconddomain.tld	user
address2@thirddomain.tld	user2

After every change you make in the file ‘virtual’, run postmap:

 cd /volume1/@appstore/MailServer/etc

../sbin/postmap hash:virtual


However, when I edit the main.cf file, and restart the service, my changes are gone.
You have to edit the file 

/volume1/@appstore/MailServer/etc/template/main.template

This template overwrites main.cf at restart.