Port 993: IMAPS
Port 993 is IMAP over TLS, the encrypted way email apps read mail from a server.
What runs on port 993
IMAPS wraps the whole IMAP session in TLS from the first byte on TCP 993. Gmail, Outlook, iCloud and most providers use it for mail apps. It is the modern default over plain 143.
On a home network
Home devices do not listen on 993 unless you run a mail server. Phones and computers connecting out to 993 is normal mail sync.
If port 993 is reachable from the internet
On a mail server, 993 open is expected and fine. The risk is password guessing against mail accounts, so strong passwords and, where supported, app passwords or modern sign-in matter more than the port itself.
How to check whether it is open on your network
If 993 shows up on your public address and you do not host email, look for a mail package on a NAS and the port forward that exposes it.
To find out whether port 993 is open on something in your home, and whether the internet can reach it, the app scans your Wi-Fi from your phone and checks your public address from the outside. It says what it checked and what it could not. Get RECON for iPhone — one-time purchase, no account, nothing leaves the device.