How to report spam that appeared in Messages on iPhone that's marked "Text Message" & thus sent via my carrier, NOT Apple, & NOT from a phone #?
This kind of thing is almost impossible to block because, as you pointed out, the e-mail address is almost always forged.  I have hundreds of e-mails blocked on my account.  That being said...
How to block someone via the Messages on the iPhone or iPad

Launch the Messages app from the Home screen of your iPhone or iPad.
Tap the conversation of the person you're trying to block.
Tap the info button in the top right corner.
Tap the name of the contact.
Tap Block Caller. ...
Tap Block Contact.

This is courtesy of https://www.imore.com/how-mute-block-and-report-imessage-spam-iphone-and-ipad
Or...

How do I stop spam texts AT&amp;T?
To report a spam SMS message received on your iPhone, you can forward the text to the number SPAM (7726). AT&amp;T will reply with a text asking for the phone number of the person who sent the spam text message. Reply with the phone number and that's it.
To block a specific email address
Sign in to your AT&amp;T Mobility account and create a Message Preferences account. Add email addresses to the block list by replying to the spam text with BLOCK in the reply message.

From... https://www.iphonefaq.org/archives/971823