There was a resolution as the Original Poster (OP) only asked:
what program or entity is marking my incoming mail subject line as SPAM. Is it Avast? How can I turn that off?
So those questions were answered, certainly as far as we could with the limited information given by the OP.
The problem with regenerating old topics is the program version will have changed a lot over a year. But the spam marking as mentioned is only in the paid versions of avast.
You also don't say what email program you use (MS Outlook used to use an avast plugin so anti spam measures were embedded into Outlook) as that may change where it is assessed ?
Since I use avast free, I don't have the anti-spam module, so can't speak from personal experience - but the settings for it are probably bound up in the AvastUI > Settings > Active Protection > Mail Shield > Customize options.