If you want to use postfix to send email from your CentOS server, using an external smtp relay with authentication, these are few steps to do to your postfix base config.
1: INSTALL SASL MODULES AUTH
#yum install cyrus-sasl-sql cyrus-sasl-plain cyrus-sasl-lib
2: MODIFY /etc/postfix/main.cf FILE adding these lines (tested with aruba smtp server):
relayhost=smtp.server.com:25
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_mechanism_filter = plain,login
smtp_sasl_security_options =
3: CREATE /etc/postfix/sasl_passwd FILE WITH YOUR AUTH DATA:
smtp.server.com username:password
4: CREATE LOOKUP TABLE
postmap /etc/postfix/sasl_passwd
5: RESTART SERVICES
#service postfix restart
6: SEND YOUR EMAIL AND CHECK /var/log/maillog
Best regards 🙂