Learn how to block SMTP traffic in Vultr. Our Vultr Support team is here to help you with your questions and concerns.

Vultr blocks SMTP

In the world of server security, blocking outbound SMTP traffic stops instances from helping with spam distribution.

Today, we will look at blocking SMTP traffic and offering practical solutions for users managing Vultr servers, including considerations for SMTP relay services, alternative ports, TLS usage, and configuring Reverse DNS (rDNS).

Blocking Outbound SMTP Traffic

Spammers often exploit cloud services to set up temporary instances for mass spam distribution, using the resources and global reach of cloud platforms. Blocking outbound SMTP traffic acts as a proactive defense against these malicious activities.

Solution 1. SMTP Relay Services

We can use reputable SMTP relay services to handle outbound email delivery. These services are often more leniently regulated by cloud providers, offering an alternative to direct email transmission from our Vultr instance.

Solution 2: Use Port 587 or TLS

Some cloud providers restrict SMTP port 25 but allow alternative ports like 587. Utilizing Transport Layer Security (TLS) for SMTP connections boosts security and is often permitted for email communication.

Solution 3: Configure Reverse DNS (rDNS)

We have to make sure our Vultr instance has a valid Reverse DNS (rDNS) record. Some email servers use rDNS to verify the sending server, adding an extra layer of security.

Opening Ports in Vultr Servers

By default, certain ports, including mail port 25, are not open on Vultr servers. Hence, we have to manually open the port if we want to make our email server work on port 25.

We can do this via one of the following methods:

• Using iptables:

  First, modify iptables rules on the Vultr server to open ports. Then, run this command to allow a specific port, e.g., port 25:

  # iptables -A INPUT -p tcp --dport 25 -j ACCEPT
• Via Vultr Firewall Service:

  We can use Vultr’s website-based firewall solution. This involves creating a firewall group via the control panel, adding desired rules, and managing packet filtering at a higher network level.
• Using Firewall Utilities (UFW, CSF):

  For users who need additional security and refined rule customization, we can use firewall utility tools like UFW and CSF. UFW, installed by default on Vultr servers, lets us open ports with this command:

  $ sudo ufw allow 25/tcp

  Please note that we have to disable the default firewall application (firewalld) for the proper functioning of CSF in CentOS 7 instances.

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Conclusion

Today, our Support Engineers demonstrated how to block SMTP traffic in Vultr.