Seeing it on your PC
A desktop mail app sends through port 587 or 465, not 25, and many internet providers block outgoing 25 from home connections. A program on your PC connecting out to port 25 on many different addresses is how spam-sending malware behaves.
Should you block it?
On a home PC nothing should need outgoing port 25. Find out which program is doing it before allowing it.
Check it on your PC
Run this in PowerShell to see what is using port 25 right now, and which program owns it:
Get-NetTCPConnection -LocalPort 25 -ErrorAction SilentlyContinue | Select-Object LocalAddress,LocalPort,RemoteAddress,RemotePort,State,@{n='Process';e={(Get-Process -Id $_.OwningProcess -ErrorAction SilentlyContinue).ProcessName}}No output means nothing is using the port at the moment. For every connection at once, paste netstat -ano into the netstat explainer.
Block it in Windows Firewall
In PowerShell run as administrator, this stops other computers from connecting to this port on your PC. It does not stop your own programs connecting out.
New-NetFirewallRule -DisplayName "Block inbound TCP 25" -Direction Inbound -Protocol TCP -LocalPort 25 -Action Block
Undo it with Remove-NetFirewallRule -DisplayName "Block inbound …", using the same name. If you depend on the service on your local network, this rule stops that too.