People reporting empty emails from me. A hacker?

Your sysinfo isn't important for this problem.  Typically one of two things has occurred.  They are either faking your email address or if you are using web mail, especially Yahoo, they have guessed your password.  While there are viruses now targeting the Mac OSX operating system, that is somewhat less likely.

Hi DaveBaldwin:
I do use Yahoo mail as my secondary email server, but Shaw.ca is my main imap mail server.
I guess I'll go change my Yahoo password right now, then wait to see what happens. Will get back to you!
TX

...I mean of course, "Shaw.ca is my main 'pop' mail server."!

HI DaveBaldwin:

Was just thinking...if not a hacker, then how would a 3rd party have gotten recipients' email addresses - some from my Address Book, but some not in my address book? Am I missing something ?

...further to last...the addresees not in my Address book are known to me, just not added to my Address Book.

That just makes it seem more likely that they got access to one of your accounts and picked up the addresses from messages in your Inbox or other folders.

Have changed all passwords. Do you think that will put this issue to rest?

I think it probably will.  There is one other circumstance where you get 'bounce' messages because they send emails to addresses that include a non-existent address and fake your address as the sender.  That causes it to bounce back to you and appear that you have sent it.  Spammers started doing that as a way to get past spam filters.

I follow you on that. Whoever it is doing this, they must really have a case against me, as they've been busy beavers all day long today: I've been getting notices by the dozen from unhappy recipients. Very weird - a few of the recipients of these fake emails are people with whom I have had no contact  for 20 years! How did they do that? Email addresses I myself had forgotten about!

Well, I've done all I can, I think - still running malware deep scans, and all passwords changed. I think I'll have to give your solution at least overnight, to see if issue is resolved.
Hang tight.

TX

So THAT'S  why I've been feeling so paranoid lately!

And it just proves what they say: "Just because you're paranoid doesn't mean they aren't watching you." !!

Dave:

I gave this issue high points because it seemed fairly serious to me (paranoia strikes deep), and so you get the full 500 points. ( I wonder where all the other experts were on this one?) I graded your answer "Average" because you seemed to get off on a slow start. You were right all along - and the issue nows seems to have stopped - but I felt a bit left to drift between comments.
Anyway....500 points is GOOD!

TX

Thanks for the points... but if I was right all along, I don't know why you say I got off to a slow start.  When you have something like this where you do the obvious thing (change passwords) and you have to wait to see if it was something else, I guess you might fell 'adrift' while waiting.

The other experts probably agreed with me and when they do agree with the responses that are posted, they don't usually add anything.  It's when they don't agree that they step in with their comments.  Since I just told you what most everyone else would have said, I'm not surprised there were no other comments.  We answer similar questions all the time.

Oh, I see. Well, what I meant by a "slow start" was your initial offering of a few different general potential issues, but without detail as to how I might distinguish the one from the other.
I thought to myself: "Well, how would I know which of these scenarios it is? More info please." In that sense, the first stab at the problem seemed slightly confusing.
But then, as we now know, I'm paranoid. ..."not that there's anything wrong with that." lol

WT

Oh, I see.  The catch is that even at this point, you can't tell exactly what was done without asking the person who did it.

Anyway, so it's good that it appears to be working now.  Just use your paranoia to watch out for future problems.