"It isn't immediately apparent how to do this. Simply add a colon (":") to the end of the server name followed by the port you want to set. For example, imap.gmail.com:993 works just fine. Make sure the checkboxes below for SSL access are checked appropriately for your server."
More on google like this with https://www.google.com/#q=windows+10+change+mail+app+smtp+port
If you are like many, why use this horrible email app? Dump it, get Thunderbird and carry on.
I don't see in the Mail App how to change the ports.
I'd need to change the SMTP port to 26 (not standard but my mail server is configured this way).
I know an e-mail client like Outlook or Thunderbird would allow it but this is not what I need at the moment.
The only option I found was SSL or no SSL, but I did not find how to pick the actual port.
Thank you for your input.