And it had an attachment ... which contained the content of the email that was sent and bounced, yeah? And it was not a message you sent?

I would do a very deep scan using whatever malware detection you have because messages being sent by (apparently at least) your pc, w/o you sending them yourself and not appearing in the sent items of outlook is fairly troubling. some malware just sends crap out in a very scattershot way, basically turning your machine into a spam factory, gathering up any email addys it can find that are on your computer, some of which you may not even know are there, and some of those are probably not valid.

I use imgur dot com for posting images I want to share online. if you use chrome there's a plugin that allows drag/drop of photos to your online imgur account, its pretty handy.

... I cc'd it to my daughter, and that's how it appeared in my SENT folder. But I got an "undeliverable" message from Outlook. The e-mail that it cited as not deliverable to was strange. The "Undeliverable" email from Outlook has an attachment that contained a copy of my sent e-mail. I opened that attachment and that's where the weird e-mail address appeared in the cc: space instead of my daughter's e-mail that I had typed in.

I'll explore imgur.com. Or Imgur Dot Com???

Dollars to donuts you accidentally put that in the cc: field yourself w/o realizing it

imgur.com, it's free file-sharing, it's legit. used for years and years.

EDIT: btw, outlook probably figured out that the bad address was bad based on the bounce, and removed it from the cc: list when you viewed the Sent folder (as it did not actually reach that destination, ergo it wasn't 'sent there' ... at least, in a manner of speaking). That's the only explanation I can think of, though I've never seen it do that personally at least not that I've noticed.

1 - Why didn't it show up in the CC field in the copy in my Sent folder?

2 - In the copy of my e-mail returned by the Outlook error message, why did my daughter's e-mail, which I DID put in NOT show up?

In other words, one e-mail showed up in the Sent folder copy and a different one showed up in the copy returned by Outlook.

Kay I didn't realize that, thought cc was just empty.

You're right there's a not a good explanation for that then ... this may've been a glitch on your daughter's email server in this case, or corruption in the transport in general ... lost packet that sort of thing.

You got a strange message,
and so you opened the attachment?

.... error message from Outlook.com.

I gotta find out how to get my screen shots to work here on DU. Not worth trying to explain in words.

If it's attaching screen shots, it's not difficult to get a URL from Dropbox. It's just that it isn't pasteable into DU. I've asked Admin about this in the past and it's a known problem. Something about the structure of the URL.

If that's not what you're talking about, what are you talking about?

Whereas the sharing source that the OP is accustomed to using (dropbox), the URL format doesn't work for this purpose on this forum.

And always have an attachment containing the original email you tried to send.

That said, you should always scrutinize the "Undeliverable" email itself (the one from the mail server, not your original) before opening the attachment (VERY well, if you don't think you sent the original in question) to make sure it looks legit, and confirm that the attachment file itself doesn't have an extension like '.exe' or '.bat'. It should be something like .txt or .html I believe in most cases.