Postfix Relayhost Setup

Cloud service providers such as Amazon often prevent email sending by default on their EC2 instances to stop spammers. This can be a nuisance when you want to send legit email. You can request for this restriction to be removed, but it can be a bit of a black box if and when they will accept your request.

Here's a quick guide how to configure your postfix with a relayhost to send emails through a third party email gateway such as Sendgrid, Mailgun, or Mandrill. The steps are for Ubuntu, but should be adaptable for most linux based systems.

Install postfix sudo apt-get install postfix
Copy and modify the config from the bottom of this post to /etc/postfix/main.cf
Replace at least relayhost and smtp_sasl_password_maps in the config with your own configuration
Create /etc/mailname file if it's not there yet and put in your preferred hostname
Restart postfix sudo systemctl restart postfix
Install sendemail sudo apt install sendemail
Test sending an email sendemail -f fromuser@fromdomain.com -t touser@todomain.com -u Subject -m "message" -o tls=no
Check your logs tail /var/log/syslog -f to see if any issues cropped up
Remember to configure the sending domain in your email service provider

smtpd_banner = $myhostname ESMTP $mail_name
biff = no
append_dot_mydomain = no
readme_directory = no

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

smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
myhostname = localhost
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = $myhostname
relayhost = smtp.mailgun.org
smtp_sasl_auth_enable = yes=
smtp_sasl_password_maps = static:postmaster@yourdomain.com:your_secret_key
smtp_sasl_security_options = noanonymous
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = loopback-only
inet_protocols = all
local_recipient_maps =