Email Server

• 
  https://cr.yp.to/qmail.html
• 
  http://www.postfix.org/
• 
  https://smtpd.github.io/qpsmtpd/
• 
  https://www.dovecot.org/
• 
  https://www.clamav.net/
• 
  https://spamassassin.apache.org/
• 
  https://www.maxmind.com/
• 
  https://letsencrypt.org/

A private open source email server provides unlimited configuration, customisation, and users, with no additional cost per user, and mailbox size up to the available storage.

• DKIM Signed Email
• DMARC Policy
• SPF Policy
• DNSBL Blocklists
• RHSBL Blocklists
• URIBL Blocklists
• Whitelist / Blacklist
• Greylisting
• Spam and Virus Filtering
• GeoIP Filtering
• Multi-Domain
• Sieve Email Filtering
• Blind Carbon Copy
• SSL Encrypted IMAP + SMTP + POP3
• TLS Transport to Remote SMTP Hosts
• SSL Webmail with DAV Integration of Calendars, Contacts, Tasks
• Open Source Letsencrypt SSL Domain Verification Certificates, or Commercial SSL Certificates.

Koozali SME Server is free and open source – there are no limitations and you can inspect, integrate, extend and modify however you want.