Postfix

From ASSPSMTP

Edit /etc/services (insert new line smtp-local after smtp):

 smtp            25/tcp          mail
 smtp-local      26/tcp

Possibly you might have to copy /etc/services for postfix

 cp /etc/services /var/spool/postfix/etc/services

Edit /etc/postfix/master.cf and change first line from smtp to smtp-local:

 # ==========================================================================
 # service type  private unpriv  chroot  wakeup  maxproc command + args
 #               (yes)   (yes)   (yes)   (never) (100)
 # ==========================================================================
 smtp-local  inet  n       -       -       -       -       smtpd

Restart postfix:

 /etc/init.d/postfix restart

Done! :-)