Verificamos que myhostname sea igual al que configuramos
myhostname = web-03-sis-ccbd.siua.ac.cr
Comentamos las lineas
#smtp_tls_security_level=may
#relayhost =
#inet_interfaces = all
Agregamos al final
# Enable auth
smtp_sasl_auth_enable = yes
# Set username and password
smtp_sasl_password_maps = static:servidores@siua.ac.cr:sa2
smtp_sasl_security_options = noanonymous
# Turn on tls encryption
smtp_tls_security_level = encrypt
header_size_limit = 4096000
# Set external SMTP relay host here IP or hostname accepted along with a port number.
relayhost = correo.siua.ac.cr:587
# accept email from our web-server only
inet_interfaces = 127.0.0.1
NOTA: Recuerde cambiar la contraseña
Reiniciamos el servicio
systemctl stop postfix
systemctl start postfix
systemctl status postfix
Verificamos que el servicio esta esperando en el puerto 25