Postfix
# apt install postfix
YES
# vi /etc/postfix/main.cf
relayhost = [smtp.gmail.com]:587
smtp_use_tls = yes
smtp_tls_CApath = /etc/pki/tls/certs/ca-bundle.crt
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_tls_security_options = noanonymous
smtp_sasl_mechanism_filter = plain
認証設定
Google アプリパスワード生成
# vi /etc/postfix/sasl_passwd
[smtp.gmail.com]:587 <gmail_address>:<password>
# postmap hash:/etc/postfix/sasl_passwd
# rm /etc/postfix/sasl_passwd
転送設定
# vi /etc/aliases
pi: username@gmail.com