Postfix

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

Steps for Ubuntu 14.04 LTS

   1 apt update
   2 apt install mailutils postfix
   3 # Choose ok, Internet Site
   4 # System mail name: bitarus.mooo.com
   5 nano /etc/postfix/main.cf

/etc/postfix/main.cf

mydomain=bitarus.mooo.com
myhostname = mail.bitarus.mooo.com
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

   1 service --status-all
   2 service postfix restart

User mail spool

   1 touch /var/mail/userx
   2 chown vitor:mail /var/mail/userx
   3 chmod o-r /var/mail/userx
   4 chmod g+rw /var/mail/userx

SASL

   1 apt install libsasl2-2 sasl2-bin  libsasl2-modules
   2 nano /etc/default/saslauthd # change START=yes
   3 

/etc/default/saslauthd

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"

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

Test starttls

   1 openssl s_client -connect 127.0.0.1:25 -starttls smtp

Remove CRAM-MD5 authentication mechanism

   1 cd /usr/lib/x86_64-linux-gnu/sasl2
   2 service postfix stop
   3 mv *crammd5* /root
   4 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.

DKIM (Domain Keys Identified Mail) + SPF

• https://www.digitalocean.com/community/tutorials/how-to-install-and-configure-dkim-with-postfix-on-debian-wheezy

DKIM is an Internet Standard that enables a person or organisation to associate a domain name with an email message. This, in effect, serves as a method of claiming responsibility for a message. At its core, DKIM is powered by asymmetric cryptography. The sender’s Mail Transfer Agent (MTA) signs every outgoing message with a private key. The recipient retrieves the public key from the sender’s DNS records and verifies if the message body and some of the header fields were not altered since the message signing took place.

   1 apt install opendkim opendkim-tools
   2 nano /etc/opendkim.conf

/etc/opendkim.conf

# create file with the following lines 
AutoRestart             Yes
AutoRestartRate         10/1h
UMask                   002
Syslog                  yes
SyslogSuccess           Yes
LogWhy                  Yes
Canonicalization        relaxed/simple
ExternalIgnoreList      refile:/etc/opendkim/TrustedHosts
InternalHosts           refile:/etc/opendkim/TrustedHosts
KeyTable                refile:/etc/opendkim/KeyTable
SigningTable            refile:/etc/opendkim/SigningTable
Mode                    sv
PidFile                 /var/run/opendkim/opendkim.pid
SignatureAlgorithm      rsa-sha256
UserID                  opendkim:opendkim
Socket                  inet:12301@localhost
####

   1 nano /etc/default/opendkim

/etc/default/opendkim

SOCKET="inet:12301@localhost"

   1 vi /etc/postfix/main.cf

/etc/postfix/main.cf

milter_protocol = 2
milter_default_action = accept
smtpd_milters = inet:localhost:12301
non_smtpd_milters = inet:localhost:12301

   1 mkdir -p /etc/opendkim/keys
   2 nano /etc/opendkim/TrustedHosts

/etc/opendkim/TrustedHosts

127.0.0.1
localhost
x.x.x.x/24

*.bitarus.mooo.com

   1 nano /etc/opendkim/KeyTable

/etc/opendkim/KeyTable

mail._domainkey.bitarus.mooo.com bitarus.mooo.com:mail:/etc/opendkim/keys/bitarus.mooo.com/mail.private

   1 nano /etc/opendkim/SigningTable

/etc/opendkim/SigningTable

*@bitarus.mooo.com mail._domainkey.bitarus.mooo.com

   1 cd /etc/opendkim/keys
   2 mkdir bitarus.mooo.com
   3 cd bitarus.mooo.com
   4 opendkim-genkey -s mail -d bitarus.mooo.com
   5 chown opendkim:opendkim mail.private
   6 
   7 cat mail.txt 
   8 mail._domainkey IN      TXT     ( "v=DKIM1; k=rsa; "
   9           "p=xxxxxx" )  ; ----- DKIM key mail for bitarus.mooo.com
  10 
  11 nano /etc/bind/bitarus.mooo.com.hosts

/etc/bind/bitarus.mooo.com.hosts

mail._domainkey    IN      TXT "v=DKIM1; k=rsa; p=xxxxx"
bitarus.mooo.com.    IN      TXT "v=spf1 a mx ip4:54.68.9.58 include:_spf.google.com ~all"

   1 service bind9 restart
   2 service postfix restart
   3 service opendkim restart
   4 # see DNS TXT records
   5 dig bitarus.mooo.com txt 
   6 dig mail._domainkey.bitarus.mooo.com txt 

Outlook

• https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/set-up-spf-in-office-365-to-help-prevent-spoofing?view=o365-worldwide#create-or-update-your-spf-txt-record

# multiple includes are allowed in the DNS TXT record 
include:spf.protection.outlook.com