### Postfix MTA ###

  1. Message Transfer Agent (MTA)
  2. Modular (SpamAssAssin)
  3. Drop-in replacement for Sendmail, as it provides a 'sendmail' binary

Note: Use 'system-switch-mail*' package to switch between Postfix and Sendmail

Tasks:
  1. Install Postfix
   a. yum -y install postfix

/etc/postfix - primary configuration directory
/etc/postfix/main.cf - primary configuration file
/etc/postfix/transport - contains routing rules for domains
/etc/postfix/virtual - contains virtual user mappings

  2. Install 'system-switch-mail' package
   a. yum -y install system-switch-mail

  3. Switch default MTA from Sendmail, to Postfix
   a. system-switch-mail

Note: The default Postfix configuration binds to 127.0.0.1:25

  4. Test local mail delivery
   a. Use 'mutt' to test local delivery

  4. Configure Postfix to receive messages from remote systems
   a. set: inet_interfaces=all
   b. set mydestinations = linuxcbt.internal
   c. service postfix restart
   d. Confirm directives using: 'postconf'
   e. Attempt to send message from LINUXCBTSERV1 -> LINUXCBTSERV4
   f. If it fails, configure MTA on LINUXCBTSERV1 to listen to routable IP
    f1. update /etc/mail/sendmail.mc
    f2. make all -C /etc/mail
    f3. service sendmail restart
Note: Ensure that 'sendmail-cf*' package is installed, in order to updated .mc files to .cf files