• How to use postfix to send mails via a public provider.
• Here an example with gmx using Ubuntu.

apt-get install postfix libsasl2-modules mailx 

• When you install postfix a configuration dialog is opened.

• internet with smarthost
• mail relay: mail.gmx.net
• mail name: hostname

• You can reconfigure postfix:

  dpkg-reconfigure --priority=low postfix

  .

• add to main.cf of postfix

/etc/postfix/main.cf

smtp_sasl_auth_enable = yes
# noplaintext # only wenn you provider doesn't support password encryption :
smtp_sasl_security_options = noanonymous
smtp_sasl_password_maps = hash:/etc/postfix/sasl_password

• create /etc/postfix/sasl_password

sudo touch /etc/postfix/sasl_password 

• Enter the necessary authentication informations in the following format “smtp.mailanbieter.de username:securepassword”.
• Create db file with postmap an then delete plain text file.

sudo postmap /etc/postfix/sasl_password 
rm /etc/postfix/sasl_password

• restart postfix

/etc/init.d/postfix restart

• Most provider check the sender address so you can't send mails under another name.
• Therefor postfix has to replace the sender address.

etc/postfix/main.cf:

sender_canonical_maps = hash:/etc/postfix/sender_canonical

• create “/etc/postfix/sender_canonical” as shown below.

user user@gmx.net
www-data user@gmx.net
root user@gmx.net

• create db file and restart postfix

sudo postmap /etc/postfix/sender_canonical
sudo /etc/init.d/postfix restart   

• Now you need aliases so that postfix know where to send mails to root.

/etc/aliases

root:   user@gmx.net

• Reload the db file
• You don't need to restart postfix

newaliases

hostname | mailx -s "root `hostname` `date`" root