I suggest creating a filter for the account that marks every message read.

There are insufficient parameters. I want to occasionally initiate a scan through all folders/sub-folders marking every message as 'read', or do it after doing a search, which is what caused the problem in the first place.

It also begs the question why certain, not all, folders/sub-folders become 'unread'. The folders showing up as 'unread' appear to be random and not based on last access date, or any other cause I can think of. I have 37 top-level folders and dozens of sub-folders.

What parameters are needed? Wouldn't a search for a non-existent value be sufficient, or am I missing something?

If I set up a filter as the following:

1 Manually Run 2 Match all of the following:

   Status > isn't equal to > Read

3 Action - Mark As Read

and the destination is the Mail folder it only runs on the top-level Mail folder and doesn't go to the 37 folders I have within Mail. The Filter Log shows no activity.

If I choose Match all messages at 2 above it greys out the ability to set the condition

The alternative is to identify a separate filter for every single Folder and Sub-Folder which is unacceptable

There should be a destination parameter of All Folders and Sub-Folders, or All Messages

Or am I missing something?

Okay. My error. A quick test gave wrong impression to me. However, there are three addons, Filtaquilla, Filter email Folders, and Filter Manager that look promising.