If you need email delivered to a specific server for selected domains you do the following:

1. As root, edit the /etc/postfix/main.cf. Find (or add) the line that starts with
   transport_maps=
   If it does not exist, add the line “transport_maps=hash:/etc/postfix/transport.
   If the line exists, add “,hash:/etc/postfix/transport” at the end of the line. Note the comma.
2. Edit/create the file /etc/postfix/transport and add a line like the following:
   destinationdomain.com   smtp:Destination.Host.Address.Or.FQDN
   where:
   “destinationdomain.com” is the domain name in the email address
   “smtp:”  you put in “smtp:”. There are other options if needed. You can check the man pages for the other options, but you will be using “smtp:” 99% of the time.
   “Destination.Host.Address.Or.FQDN” is either the ip address or the name of the server that you want the email to be explicitly delivered to
3. execute as root: postmap /etc/postfix/transport
4. execute as root: service postfix restart

To add additional domain names, repeat only steps 2 and 3

Delete all email destined for a selected domain or email address

Repeat the above – but add the following line(s) to the /etc/postfix/transport file

domainWeDontLike.com    discard
OneSelectUser@someDumbDomain.com  discard

The first line will discard all email destined for AnyUser@domainWeDontLike.com.  The second line will discard all email to OneSelectUser@someDumbDomain.com. Any other email addressed to a user @someDumbDomain.com will be delivered.