Port 587: SMTP submission
Port 587 is where email apps log in to send outgoing mail.
What runs on port 587
Mail providers accept messages from their users on TCP 587, after a login, and then deliver them onward over port 25. The connection starts in plain text and upgrades to TLS with STARTTLS. Gmail, Outlook, iCloud and nearly every provider support it.
On a home network
Home devices do not listen on 587. Your phone, laptop, printer or NAS connects out to a provider on 587 when it sends email or alerts, which is normal outgoing traffic.
If port 587 is reachable from the internet
On a mail provider's server, 587 is supposed to be open. On a home network nothing should be listening there unless you run your own mail server. Exposed submission servers are brute forced to take over accounts that can then send spam.
How to check whether it is open on your network
If an outside port scan shows 587 on your public address, look for a mail server running on a NAS or PC and a port forward pointing at it.
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