I'm running the standard Mail Server but MailPlus Server should be similar.
In general for mail servers you define at least one domain (FQDN) that tells the server it has to accept and process mail destined to this domain, all other domains will be relayed on to another SMTP server.
In Mail Server this domain is set using Hostname in the STMP page. Alongside this field is a button Additional Domain where more domains can be added, and the server will also accept and process mail for these too.
Now that the mail server has received the the messages to the two domains it will look at the recipient, e.g. [email protected] and [email protected] The @fredbert.com and @bertfred.com domains are no longer important. Provided there is an account defined on the NAS as 'fred' then all [email protected] will end up there.
That's the theory, I only have one domain I need to use locally, and it worked on other SMTP servers.
As @Shadow said, you need to have DNS configured so that @fredbert.com and @bertfred.com domains have MX records that resolve to the same destination.
BTW thanks for making me look! I set Mail Server up before I had LDAP for other reasons. I had thought Mail Server didn't support LDAP users but I now see it uses Local or LDAP for accounts. Unlike MailPlus Server, Mail Server stores messages and filters in the local user's /home and is visible in File Station ... high risk of a user tidying up these folders to Trash.
How I have it set up right now with DNS records is mail.domain1.com and mail.domain2.com going to the same IP... my Rackstation.
On the Rackstation, I had the Application Portal address set to mail.domain1.com. This worked well for Domain1. I thought that if I set a Reverse Proxy of mail.domain2.com to forward to mail.domain1.co, it would work. It did not.
What I ended up doing is changing the Application Portal to a port and setting up Reverse Proxies for both domains. This seems to be working quite well.