Why doesn't the "Received: ...by" email header show the port?

I've had a problem recently with spam coming in with an envelope-from consisting of my own address. At first I thought this meant that my password had been compromised, mistakenly thinking that the "with esmtp (Exim 4.87)" in the "by" clause of my email server's Received header meant that it came over an authenticated connection. My email provider quickly corrected me, that incoming messages come from MTAs, and MTAs do not authenticate when passing along a message. So it could just as easily (and most likely did) come through an open relay, as the IP addresses were all listed on 15 or more blacklists.

When I send mail from Thunderbird, I connect on port 587, and that port requires authentication. If I could look at the headers and see that the connection to my ISP's server was on port 587, then I would know that the sender authenticated, and if it had my own address as its envelope-from, then I would know that my password had been compromised, right? Well I guess any server in the trace can modify the earlier headers, but in these cases there aren't any other servers in the trace. And even if there were, it's not clear why a server would want to modify the port number given in an earlier header.

I tried asking the email provider if they could add the port to their "Received by" clause and didn't get a meaningful response. I googled a bit to see if sendmail had an option to do this but got nowhere. However, I guess the header actually tells me they are using Exim rather than sendmail, and this link shows where the header content is configured in Exim, though it doesn't say how to refer to the incoming port number in the text.

But given that the port number seems to provide some very valuable information to anyone dealing with spam from spoofed senders, I wonder why it isn't included by default.