Internet-Security.com TCP port 109 details

TCP port 109 is the well-known port for POP2 (Post Office Protocol v2), an obsolete email retrieval protocol.
A real-world implementation was the University of Washington ipop2d daemon (part of the imap-uw package), which shipped with many Unix/Linux distributions in the 1990s–2000s.
Admins commonly enabled it via inetd/xinetd on systems such as early Red Hat and Debian releases, and it can still be found on some legacy FreeBSD or Solaris installs that kept ipop2d around.
Modern mail servers and clients have long moved to POP3 (port 110) or IMAP (ports 143/993), so live POP2 on 109/tcp is now rare.
Attackers sometimes probe 109/tcp because old POP2 daemons had known remote vulnerabilities; leaving it enabled today is generally a security risk.

TCP 109