Port 25: SMTP
Port 25 is SMTP, the port mail servers use to deliver email to each other.
What runs on port 25
Every mail server that receives email for a domain listens on TCP 25, and servers hand messages to each other over it. It dates from 1982 and was originally unencrypted, though most servers now upgrade to TLS with STARTTLS. Email apps are supposed to send through 587 or 465 instead.
On a home network
Home devices almost never listen on 25. A NAS running a mail server package is the usual exception. Printers and cameras that send email alerts connect out to a mail provider, usually on 587 or 465.
If port 25 is reachable from the internet
On a real mail server, 25 open to the internet is required and normal. On a home connection it is unusual, and a misconfigured mail server that relays mail for anyone gets abused for spam quickly. Many residential ISPs block port 25 to stop infected home computers from sending spam directly.
How to check whether it is open on your network
If you do not run a mail server, nothing on your network should have 25 forwarded; check your router's port forwarding list. A port scan of your public address from outside confirms it.
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