FIXED
The problem is fixed. I haven't had mail mysteriously disappear since May, when I made the changes listed under "WHAT I'VE DONE", below.
WHAT I DIDN'T REALIZE
I didn't realize that Opera has built-in spam filtering software that is independent of, and separate from, the spam filtering software my webhost provides on my mail server. I had already setup my webhost's spam software. Then, when I setup Opera's software, good mail started disappearing into my Spam folder.
WHAT I'VE DONE
I turned off Opera's built-in spam filter. All Messages > right click the Spam mailbox > Properties... > Rules > Internal filter > Off. Then, Options > unchecked "Learn from labeled messages".
(For the benefit of those reading this who want to learn, Opera's spam software is built into the Opera browser. You activate it in the Properties of the Spam mailbox that appears under All Messages. All Messages is in your Mail panel. If you don't see the Mail panel when you're reading your mail, hit F4. If you still don't see your Mail panel, hit Shift F12 > Panels > checkmark Mail. That'll bring up the Mail panel. Then, if you don't see a Spam mailbox in the Mail panel, click the wrench to the right of the Compose heading, and checkmark All Messages. Now, right click the Spam mailbox, and click Properties.... In the Spam dialog box, on the Rules tab, change Internal filter to Medium or Strong. Then, go to the Options tab, and checkmark "Learn from labeled messages". -- Or, if like me you want to turn it off, change Internal filter to Off.)
I also disabled local spam filtering in this specific e-mail account. Opera > Mail and Chat Accounts... > double clicked [my IMAP account] > IMAP tab > checked "Disable local spam filter for this account (server-side filtering)".
THANKS
I thank you guys, (and gals), for your help!