Yes.

In your place, I would first try to get off mailing lists, then use a block list at sonic.net. Blocking at the server is better than filtering in Thunderbird. Not having to deal with the messages at all is the best.

So, "Graymail" is really the same as "Junk" or "Spam"? Most email services are set up so stuff in Spam will be deleted after some period of time. Trash, too. So you could leave them there and they will go away eventually on their own. If you want Graymail to disappear for good immediately, without going to Trash, hold down the Shift key while you delete them.

There are Account Settings that will automatically delete Junk mail, immediately, or after some period of time. There are global settings that do similar things. If Sonic injects some Graymail-identifying information into the header, you might also create Filters that send them directly to Trash.

My ISP (Sonic.net) creates a "Graymail" folder that I see within my TBird Inbox. Functionally it is equivalent to Junk, since I've seen few or no false positives. The shift-key method may be one answer after "select all" to save a step. Also, I see an option for "Customize header" when creating or editing a filter, but I'm unclear on how to use it; is there a help page for this?

Is there a reason to customize the headers? Don’t you just want the messages deleted?

https://support.mozilla.org/en-US/kb/organize-your-messages-using-filters

An earlier response of mine has not been published yet. I do not know why not.

I don't want to modify any headers; but if I can use one or more to identify junk (say, a Spam Assassin score), that could be a useful filter criterion.

I don't want to modify any headers; but if I can use one or more to identify junk (say, a Spam Assassin score), that could be a useful filter criterion.

Oh, "Customize header" may help you do that. I am not familiar with it. It seems to let a user add a header that is not listed to the filter's selection criteria.