This article covers some tips to secure a mail server.
Email on the internet is sent by the Simple Mail Transfer Protocol (SMTP). Where a mail flow between servers is not encrypted, it could be intercepted by an ISP or government agency and the contents can be read by passive monitoring.
Basically, When emails are sent between two parties, unless BOTH parties use encryption the message is open and can be read by anyone who intercepts it.
Any emails sent to and received from mailboxes that only send cleartext emails should be considered as security liabilities.
Tips on how to secure your mail server:
1. Encryption: When securing your mail server, make sure you are using secure connections. Encrypt POP3 and IMAP authentication and use SSL and TLS.
2. Mail relay configuration: Avoid being an open relay for spammers by specifying which domains/IP addresses your mail server will relay mail for.
3. Connections and default settings: To avoid DoS attacks, 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.
4. Access Control: To protect your server from unauthorized access, implement 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.
5. Abuse prevention: Check DNS-based blacklists (DNSBLs) and reject email from any domains or IPs listed on them. Check Spam URI Real-time Blocklists (SURBL), and reject any messages containing invalid or malicious links.
Also, maintain a local blacklist and block any IP addresses that specifically target you. Employ outbound filtering and use CAPTCHA/reCAPTCHA with your web forms.