Postfix

Postfix is a very popular open source Mail Transfer Agent (MTA).

Steps for Ubuntu Ubuntu 14.04 LTS

apt-get update / apt-get install mailutils
apt-get install postfix
Choose ok, Internet Site
System mail name: bitarus.allowed.org
nano /etc/postfix/main.cf

mydomain=bitarus.allowed.org
myhostname = mail.bitarus.allowed.org
smtpd_recipient_restrictions=permit_sasl_authenticated , reject_unauth_destination
relay_domains=
smtpd_sasl_auth_enable=yes
smtpd_sasl_security_options=noanonymous
smtpd_sasl_local_domain=$myhostname
broken_sasl_auth_clients=yes
smtpd_relay_restrictions=permit_sasl_authenticated
home_mailbox=Mailbox
mail_spool_directory=/var/mail

service --status-all
service postfix restart

User mail spool

touch /var/mail/userx
chown vitor:mail /var/mail/userx
chmod o-r /var/mail/userx
chmod g+rw /var/mail/userx

SASL

apt-get install libsasl2-2 sasl2-bin libsasl2-modules
nano /etc/default/saslauthd # change START=yes

START=yes
PWDIR="/var/spool/postfix/var/run/saslauthd"
PARAMS="-m ${PWDIR}"
PIDFILE="${PWDIR}/saslauthd.pid"
DESC="SASL Authentication Daemon"
NAME="saslauthd"
MECHANISMS="shadow"
THREADS=5
OPTIONS="-c -m /var/spool/postfix/var/run/saslauthd"

dpkg-statoverride --force --update --add root sasl 755 /var/spool/postfix/var/run/saslauthd
echo 'pwcheck_method: saslauthd' > /etc/postfix/sasl/smtpd.conf
usermod -a -G sasl postfix
service saslauthd start

Test starttls

openssl s_client -connect 127.0.0.1:25 -starttls smtp

Remove CRAM-MD5 authentication mechanism

Go to folder /usr/lib/x86_64-linux-gnu/sasl2
service postfix stop
mv *crammd5* /root
service postfix start

In python 2.7 the CRAM-MD5 is a preferred authentication method for login in smtplib. The current postfix advertises CRAM-MD5 but does not accept it if smtplib tries to authenticate with it. So CRAM-MD5 authentication mechanism was removed.

AWS restrictions to send outbound emails - port 25

https://aws.amazon.com/premiumsupport/knowledge-center/ec2-port-25-throttle/

AWS blocks outbound traffic on port 25 (SMTP) of all EC2 instances and Lambda functions by default. If you want to send outbound traffic on port 25, you can request for this restriction to be remove

https://docs.amazonaws.cn/en_us/AWSEC2/latest/UserGuide/ec2-resource-limits.html#port-25-throttle

On all instances, Amazon EC2 restricts traffic on port 25 by default. You can request that this restriction be removed. For more information, see How do I remove the restriction on port 25 from my EC2 instance? in the AWS Knowledge Center.