When I think about email in Linux, the first word that comes
into my mind is definitely Postfix. Postfix basically is a free and open
source MTA (Mail Transfer Agent), which simply routes email messages. So by
using this open-source software you can implement a robust and highly reliable
SMTP server.
So now I’ll try to install and configure Postfix on my Centos 7 server.
To start let’s get and install the packages. (In the latest
releases of Centos Postfix is pre-installed).
#yum -y install postfix
After the rpm deployments we can spot our configuration directory
at /etc/postfix , master.cf and
main.cf are our main configuration files.
At the moment, for a basic configuration, we’re only
concerned about main.cf, use your favorite editor to open this file (mine is vi
editor and I get a lot of trolling from my fellow linux admins about this).
So to start we need to specify a few parameters, locate :
myorigin directive,
at the SENDING MAIL section of the conf, here we just need to give the machine’s
hostname.
mydomain at INTERNET
HOST AND DOMAIN NAMES, where we specify the local domain suffix of the machine
eg:mydomain.local.
myhostname where we
specify our internet hostname address, this address must refer to a register
domain, eg:myhostname.mydomain.com
mynetworks under TRUST
AND RELAY CONTROL where we specify the ip addresses we want to allow email
relay from. Those must be written in a pattern of ip/network mask eg:
10.0.100/24.
Now we may have some nasty ip addresses in our allowed
networks spamming our server, so to prevent this we must have a blacklist to
put them inside. For this we need to add the following:
smtpd_recipient_restrictions =
check_client_access hash:/etc/postfix/blacklist
permit_mynetworks,
Now we have a blacklist file where we can block senders.
Well, actually we have the reference and not the file, to
create this file give :
# touch /etc/postfix/blacklist
And to make the file active give:
# postmap /etc/postfix/blacklist
Now we can block a sender by editing the file and simply
adding
“sender’s Ipadress” REJECT
It seems we’re all set, so to start our server we need to :
#systemctl start postfix
Althought
#service postfix start
can still do the job
and don’t forget to make the service initializing on system startup
#systemctl enable postfix
So now we have an all working SMTP alive and we can check the
status
# systemctl status postfix
Server is able to receive connections (port 25). Now we can
watch the incoming email messages flowing in and out and monitor the relaying process
by having a look at the logs:
# tail –f /var/log/maillog
Sometimes there are emails which can’t be send for various
reasons, and are stuck in the server’s queue, we can list them by giving:
#mailq
and we can examine those emails by opening them, in text
format of course
#postcat –qv “queuefilename”
Or get rid of all of deferred emails in the queue
#postsuper –d ALL deferred
