Unfortunately that is a flaw in email. As for why your provider does anything, ask them.

In Thunderbird use Junk controls.

I suggest you use ctrl+u to inspect the source of some of these messages, and look into the origin as indicated by the servers that handle the messages.

What I expect you to discover is that the "from" address bears little or no relation to the actual origin of the message. The vast majority of "from" addresses in email are falsified. Why would a spammer tell you who sent it?

There are challenge based systems, whereby if you send a message, a reply is sent back to you, asking you to confirm you sent it. Your email address then goes on to a trusted sender's list. There is some of argument about whether such systems do more harm than good. They do not seem to have caught on in any big way.

As to how or why an ISP or email provider should, or could, intervene. If, say, a nuisance in China sent a message to you at your gmail account, but using an earthink address in the "from" field, how would earthlink get to even look at this message, let alone do something with it?

Do your filters catch much? Filtering on "from" addresses tends to be somewhat futile, since it's uncommon to have messages sent repeatedly by the same ostensible senders.

Thanks for the tip on using control-U. The email addresses from the sender are formatted correctly, but they do not exist and an attempt to ping them or use an app to verify them always fails. The only point I am making here is that unsolicited emails are arriving in my Inbox at Earthlink that are from non existent email addresses. I would like to think that Earthlink or any other ISP email provider could send such emails directly to my Web Mail spam folder or just not deliver them. I do not know a whole lot about it, I am just asking questions and trying to find a solution.