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Mail server security basics

A mail server is a computer within your network that is responsible for sending, delivering, and storing electronic mail messages. If your mail server is not secure, malicious actors can gain unauthorized access to exploit it by sending different types of spam and phishing messages from your mail server. If you are hacked, there are multiple risks - each having a different impact on your email program:

  • Spam creates a lot of complaints against your domains and IP addresses, which can result in your mail being blocked at major ISPs.
  • Spam can lead to a drop in subscriber engagement with your legitimate email.
  • Both subscribers and mailbox providers could block your email.
  • Malicious actors will likely send spam to random email addresses that are not your vetted subscribers, which usually include a high number of spam traps.
  • If unauthorized content is being sent from your infrastructure and you are hitting spam traps, then you will likely be listed on publicly available blacklists.
  • The time it takes for performance metrics to normalize varies based on the scale of the spam run, but no matter how long it takes, it will have a detrimental effect on email deliverability and ROI.

Mail server security basics

  • Encryption: Secure your mail server by making sure you are using secure connections. Encrypt POP3, IMAP authentication, and use SSL or TLS.
  • Authentication: Check inbound email to detect spoofed mail using SPF, DKIM and DMARC.
  • Mail relay configuration: Avoid being an open relay for spammers by specifying which domains and IP addresses your mail server will relay mail for.<
  • Connections and default settings: Limit the number of connection and authentication errors that your systems will accept. Remove unneeded server functionality by disabling any unnecessary default settings. Have a dedicated mail server and move other services like FTP to other servers. Keep total, simultaneous, and maximum connections to your SMTP server limited.
  • Access control: Protect your server from unauthorized access, by implementing authentication and access control. For example, SMTP authentication requires users to supply a username and password to be able to send mail from the server. Make sure access to your servers is on a need-to-have basis and is shared with as few people as possible.
  • Abuse prevention: Check DNS-based blacklists (DNSBLs) and reject email from domains or IPs listed on them. Check Spam URI Real-time Blocklists (SURBL), and reject messages containing invalid or malicious links. Also, maintain a local blacklist and block IP addresses that specifically target you. Employ outbound filtering and use CAPTCHA/reCAPTCHA with your web forms.
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