Apunte de cómo configurar Postfix como relay de GMail. Funcionó en Ubuntu Server 14.04 LTS x64, 16.04 LTS x64 Proxmox VE v5.0 (Debian Linux x64)
# apt-get install postfix mailutils libsasl2-2 ca-certificates libsasl2-modules # nano /etc/postfix/main.cf
smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
smtpd_use_tls=yes
smtpd_tls_session_cache_database = btree:{data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:{data_directory}/smtp_scache
inet_interfaces = all
relayhost = [smtp.gmail.com]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt
smtp_use_tls = yes
# nano /etc/postfix/sasl_passwd
[smtp.gmail.com]:587 USERNAME@gmail.com:PASSWORD
# chmod 0400 /etc/postfix/sasl_passwd # postmap /etc/postfix/sasl_passwd # service postfix restart # echo "Probando uno, dos" | mail -s "postfix relay" usuario@dominio.xxx
