If you have added Reply-to as a custom header in the filter rules, it is also available in Classic Search (Ctrl+Shift+F), which lets you test searching for messages before using it in a filter. What exactly is the rule, e.g. Reply-to + contains + some text? Try different choices for 'some text' to see if a shorter string is more effective.

I would also suggest you apply the spam filtering, if it's available, on your mail provider's webmail site.

Thank you, sfhowes, for your reply.

At your suggestions, I tried Classic Search (Ctrl+Shift+F) and used the same rule I customized in my Thunderbird Message Filter: Reply-to + contains + @gtin.matarovilla.icu (which is the domain that appears on every email from this spammer, in a "Reply to" line below "Subject"). In Classic Search, after hitting "Search", the latest spam email from this guy showed up right away...

...which seems to indicate that this exact same rule should work in my Message Filter as well, right? Except it doesn't.

As for trying a shorter string to see if it is more effective, I've tried "matarovilla", "matarovilla.icu", "gtin.matorovilla.icu", and the string above, which includes the @ symbol. None have worked.

My email provider, Spectrum, has "a built in spam filtering system", but this is apparently not helping.

Is there some third party software, or Thunderbird add on, that might solve this dilemma? I've done some light research on the few that show up in a Google search, but they don't specifically seem to address this kind of problem.

Got another spam from the same spammer while I typed this!

[I mean, what is their thought process anyway? That by irritating the hell out of people they're going to generate sales?? Grrrrr...]

[BTW, just for the record, I have not responded, replied, or clicked on any link in these emails. Thunderbird, at least, DOES "think these are junk mail". I then delete them.]

I don't know why search would produce results but the filter doesn't. Maybe the choice of before/after junk classification in the filter makes a difference. Enable the filter log and see if it gives any clues, and run the filter manually.

You can enable TB's junk controls in Junk Settings under Account Settings. It is supposed to be more effective than filters, but some users find the opposite.

As you suggested, I tried changing the choice of "Filter before junk classification" to "Filter after junk classification"... and it made no difference. I still received a handful of spam from the same guy after I made the switch.

I've now enabled the "Filter Log" and selected the "Inbox" drop-down option on the email address that's getting spammed. We'll see if it reveals anything useful. [Having just enabled the filter log, when I "Run Now", nothing yet appears in it.]

My junk controls in "Junk Settings" (under "Account Settings") already show "enabled" for the email in question. [Though I was a bit confused by this suggestion itself: Did you mean that if "Junk Settings" are enabled, it DISABLES message filters? Because if that's the case, that might be the problem!]

Finally, I think what it really boils down to is something other threads on this topic in Thunderbird Support have already mentioned: It seems that a "customized" message filter heading (like "Reply-to") just doesn't work in message filters. No one ever resolved this in any previous thread, so maybe I'm just having the same problem. Which kind of begs the question: If true, why wasn't this corrected? Why continue to provide the option for a "customized" message filter heading, if they don't work?

[And it's pretty obvious my "Reply-to" customized message filter is not working. I even tried adding a new rule that said: Reply-to + doesn't contain + (gibberish). Since no one ever uses a "Reply to" heading in any emails I receive, this should have blocked the spammer as well. But it didn't.]

Perhaps another fix would be adding an option to block a domain that appears anywhere in the header? This, it seems, would do the trick, too.

Or maybe I'm just grasping at straws. Pretty frustrating, having the means to block this nuisance staring me in the face (his "Reply to" domain name remaining constant), but unable to implement it.

Thanks again, though, for your help.

Don't look at the Reply to in the Header Pane, but in the message source (Ctrl+U) headers. At least one user manages to filter on Reply-to:

https://support.mozilla.org/en-US/questions/1273741#answer-1269459

As stated before, test the filter rules with Classic Search before applying them in a filter.

Thanks for the fast reply.

I've done some testing, and I have some good news/bad news:

Good news: My "Reply-to" message filter apparently DOES work (as you suspected, having wondered why it worked in Classic Search, Ctrl + Shift + F). Woo-hoo!

Bad news: It doesn't "Delete from POP server" which is what I have selected for "Perform these actions".

This is how I know, i.e., tested it. I added a second action to the above, namely, "Delete Message". When I tried "Run Now" with the message filter, it deleted the spam messages, i.e., put them in my Trash folder. This means the filter WAS working after all.

However, my intent was always to have these spam messages deleted at the server, so I never have to see them. To see if that works, I did another test. I created a test message filter with a "Subject" that contained a test phrase. I then send an email from a different account with that test phrase in my Subject heading. Sure enough, the message filter flagged it, and put it in my Trash folder (deleted it)...

...but, again, the message filter did NOT "Delete from POP server", which was one of the two actions specified.

Unless I'm misinterpreting what "Delete from POP server" means?

I had always thought it meant that filtered messages were removed before they ever reach my Thunderbird Inbox? In the past, after I added a domain name to my "Block List", I never saw another spam message from any of those domains again.

So I guess my question now is: Is there a way to have this spam (which is being recognized by my message filter) deleted at the server BEFORE it reaches my Thunderbird Inbox?

What delete on server does depends on the pop server implementation.

On Outlook/Hotmail, it will move the mail to the POP folder. On Gmail is will be ignored unless you modify the default settings on the gmail web site. Yahoo probably have undocumented features, but claim to do what the mail client requests.

The big unresolved in filters is they only act on the first occurrence of a header field. So things like Received: are useless as they almost always appear more than once and in the case of Received the last entry is delivery to you.