Dr Teeth wrote:

That is not the issue...things are filtered to the correct folders
this end.

And what are the rules that perform this filtering?

It is the replying that is the problem. Sometimes Outlook sends a
reply to a line of correspondence (that has involved account B
throughout, for e.g.) from another account, for no reason that I can
see.

Have you added the Account column to the display to make sure the account
you think is the correct one is the one Outlook thinks is the correct one?

If separate accounts on the ISP's server will fix the issue, it looks
like it will be easier than I thought. It would still be easier if I
did not have to bother though g.

I receive multiple Yahoo! accounts (separate mailboxes on Yahoo!'s servers)
and have never seen the problem you describe.
--
Brian Tillman