Exim

To block any outgoing email from a particular local user, you can use the acl_not_smtp to discard or reject messages that aren't sent via SMTP.

Up near the rest of the acl definitions:

acl_not_smtp = acl_not_smtp

And then somewhere in the acl section:

acl_not_smtp:

 discard condition = ${if eq {$sender_address}{localuser@wood.bocks.com}{yes}{no}}
         message = $sender_address is not permitted to send messages
         log_message = $sender_address is not permitted to send messages

 accept

Using deny instead of discard sends a local bounce back to the user containing this message and then their original message.

Subject: Mail failure - rejected by local scanning code

A message that you sent was rejected by the local scanning code that
checks incoming messages on this system. The following error was given:

 localuser@wood.bocks.com is not permitted to send messages

------ This is a copy of your message, including all the headers. ------

Disable sending to specific IPs

Add to the appropriate router, or make a new one.

You can use this option to disable the use of IPv4 or IPv6 for mail delivery by means of the first or the second of the following settings, respectively:

ignore_target_hosts = 0.0.0.0/0
ignore_target_hosts = <; 0::0/0

The pattern in the first line matches all IPv4 addresses, whereas the pattern in the second line matches all IPv6 addresses.

For example, to not use IPv6 to send to gmail or googlemail users, add a new router before the dnslookup router:

google:
 driver = dnslookup
 domains = googlemail.com : gmail.com
 transport = remote_smtp
 # No IPv6
 ignore_target_hosts = <; 0::0/0  

dnslookup:
 driver = dnslookup
 domains = ! +local_domains
 transport = remote_smtp
 ignore_target_hosts = 0.0.0.0 : 127.0.0.0/8
 no_more

Switching MTA in CentOS 6

Install exim/sendmail/postfix/whatever.

Choose from the list using the alternatives program.

# alternatives --config mta

There is 2 program that provides 'mta'.

  Selection    Command
-----------------------------------------------
*+ 1           /usr/sbin/sendmail.postfix
   2           /usr/sbin/sendmail.exim

Enter to keep the current selection[+], or type selection number: 2
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