Blocking or blacklisting entire Top Level Domains (TLDs) used for hostname in email server setup can be a very effective solution to stop low reputation mail servers delivering mails to your email system(s).
As most email providers use common TLDs like .net, .com
or .de in their email setup, you can selectively block new or low reputation
TLDs and this way reducing a lot of unwanted emails.
Blocking or blacklisting entire Top Level Domains (TLDs) in Postfix mail server may have a big impact to reduce spam, phishing, and other unwanted emails. Dependent on the use case of your systems it is a valid and effective solution. The only drawback is, to monitor your email traffic and check, whether valid emails from blocked TLDs are sent to your systems. Blocking a whole TLD is mostly a temporary thing, as the bad guys move on if the domains on a specific TLD are not usable for their cases.
Sending a test email from shell using command line, send mails in shell script is a common task for an email admin. Mails were and still are used to inform admins regarding changes, warnings and problems.
Dependent on the task you want to achieve, some content should be in the body of an email or attached as file. Let’s figure out, how to do it using Linux or Unix command line like bash or zsh.
You have built your local IP blacklist for Postfix. Invested time to analyze server logs or Rspamd history to blacklist single hosts, IP addresses, or even whole networks. But how well is it working compared to public blacklists you are using? Time to see the results and get some visual insights through Munin Monitoring System.
First of all, you have to decide how to visualize the data in your graph. There are different solutions you can approach. You may want so kind of overview, to see how many hits your blacklist has. Maybe you want to monitor or compare the results to public or payed blacklists you are using.
By running a Postfix mail server, you need to know some commands to manage Postfix mail queues. With this knowledge, you can manage queues and write simple scripts to accomplish larger tasks.
First, I will list the most relevant commands you need for your daily business. Then we will build up real-life examples to understand how these commands work, how to use them, and when they can harm.
You know those same hosts, providers, or whole networks, who get through your spam filter and spam your users with the same content over and over again. By building up your blacklist in Postfix, you can stop them from bothering you and your users. We will explain how local blacklists work in Postfix and how to use them with whitelists to have better control and reduce errors.
Blocking other hosts must be a well-thought decision because blocking single hosts, IP addresses, or even whole networks will stop all delivery attempts to your mail servers.
If you use any blacklists for email services, whitelisting important Hosts, single IP Addresses, and even whole networks is an essential task. By whitelisting valuables email services to your users or even customers, you as admin won’t reject important email communication.
If you use any blacklist service or has your blacklist in your Postfix setup, you will probably find similar logs:
root@server $ grep "blocked using" /var/log/mail.log
Dec 17 4:07:18 server postfix/smtpd[21213]: NOQUEUE: reject: RCPT from
unknown[1.2.3.4]: 554 5.7.1 Service unavailable; Client host [1.2.3.4]
blocked using ix.dnsbl.manitu.net; Blocked - see http://www.dnsbl.manitu.net/lookup.php?value=1.2.3.4;
from=<x0h0ihbxazf@somedomain.example> to=<cp57ouwn7zm9@mydomain.example> proto=ESMTP helo=<[1.2.3.4]>
As you can see in this example, IP Address 1.2.3.4 was blacklisted by a third-party service.
Monitoring email queues on your favorite email server is an essential task of an email admin, as rising queues are often an indication of problems. First, you need to understand how queues work, manage them, and get emails enqueued. Then build a shell script to monitor them. In the last step, you will learn how to get smartphone push notifications by sending emails to a third-party push service.