In /etc/postfix/main.cf, with TLS authentication on port 587.

relayhost = mysmtp.domain.com:587
 
smtp_use_tls = yes
smtp_tls_security_level = encrypt
 
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
 
sender_canonical_maps = static:authorized@domain.com

relayhost is the SMTP server you want to use. It must appear exactly as it is in /etc/postfix/sasl_passwd (all the ] and [ must be the same).

smtp_tls_security_level must be encrypt for port 587 and may for port 25.

sender_canonical_maps might not be respected by the SMTP server, changed on the fly or even bounced discarded. This depends on the provider.

After editing main.cf, do not forget to:

service postfix restart

Documentation: http://www.postfix.org/SASL_README.html