Internet-Security.com TCP port 25 details

Port 25/TCP is used by the Simple Mail Transfer Protocol (SMTP) for server-to-server email delivery; common MTAs listening on it include Postfix, Exim, Sendmail, qmail, OpenSMTPD, Microsoft Exchange Server, hMailServer, and PowerMTA.
Large hosted email services accept inbound mail on port 25: Google Workspace MX (aspmx.l.google.com), Microsoft 365/Exchange Online Protection, Amazon SES inbound email, Fastmail, and Proton Mail gateways.
Email security gateways terminate SMTP on port 25 to filter and relay mail: Cisco Secure Email (IronPort/ESA), Proofpoint Protection Server, Barracuda Email Security Gateway, Mimecast, and Trend Micro Email Security.
Many enterprise mail servers and appliances negotiate STARTTLS on port 25 for opportunistic SMTP encryption.
Port 25 is also used by on-prem groupware/mail platforms like HCL Domino and Zimbra for their SMTP/MTA components.
Security/abuse: Attackers probe port 25 for open relays and misconfigurations to send spam/phishing; notable SMTP-reachable vulnerabilities include Exim CVE-2019-10149 and OpenSMTPD CVE-2020-7247.

TCP 25