My Apple Mail misbehaving w/ incoming mail. Any Clues?

I have had my main email address for nearly 20 years. I use an iMac, with my email address being s*@mac.com for years. (Now, the .mac is an alias for iCloud).

Anyway, over the last 3+ years, my Apple mail began to file incoming mail in a weird, haphazard, unorganized way, like putting email about medical issues–for example–into random mailboxes instead. I am at wits end!

Incoming mail also gets held up often, for many days, even a week, sometimes more. It does the same on my iMac, iPhone & iBook Air.

I installed Spam Sieve soon after this started, and it worked at first, but it failed after I forgot to upgrade my system at the same time I updated (or upgraded) Spam Sieve. It has not worked since. I am at wits end. Sadly, I am now using a google address, because I need my mail!

HELP!