I would start by changing your email password. I've encountered hijacked accounts that subsequently blast out spam and what you may be seeing are the undeliverables. It could certainly be something else, but that's where I'd start.

I would also stop opening them, you are giving the sender notice that they have found a live address perhaps. If you use Outlook Express stop using the preview feature too.

Sometimes spammers manage to send spam using real email addresses that aren't theirs, and spam that goes to non-working email addresses is bounced back to you as undeliverable. But it doesn't necessarily originate from your computer, or mean your email account has been entered, nor are the emails sent to the addresses in your address book. I've had this happen to me a couple of times in the past and it stops after a day or two. However, I'd be concerned about a virus infection if any addresses were recognizable from my address book, particularly addresses that are valid. I suggest you do a virus scan on your computer and another scan with the free program Malwarebytes. Also, call a couple of your friends asking if they got any spam or other mail from you recently. (Don't email them about this: if you have a virus infection on your computer, you could be sending out a potential virus infection each time you send an email.) What is going out in these messages could be spam or it could be a virus infection waiting to happen for unsuspecting recipients. Lastly, what is the extension at the end of the hyperlink?

Here is a link that might be useful: Malwarebytes

is this a web based email account like gmail, yahoo, hotmail etc or is it a pc based email like outlook?

I would definitely run an updated malwarebytes scan right away. then I would also run an RUBOTTED scan here is that link.
RUBotted

here is a link for directions to malwarebytes, run the FULL scan not the quick
How to download and install Malwarebytes' Anti-Malware application for Windows computers

I have followed all the previous recommendations and the scans have shown nothing wrong. No returned messages as of today. Thanks for all the help. I've always used malwarebytes prior to this.
ABH

fwiw
Yahoo mail wouldn't allow one computer to send webmail and I was asked to look at it.

Any mail she tried to send, Yahoo reported back unable to send do to too many or multiple recipitants... even though she had only one name in the To: field.

It apparently was one of the add-on tool bars spamming & she had many.

I opened up her viewing area by unistalling many tool bars with revo advanced... mail worked everytime.
She never used any of those tool bars she said.

She could see way more real-estate in her browser window for sure.. She commented how she was impressed - it boots up so fast and the browser opens so fast now.
I hate tool bars.

After that she scanned and called me back with what to do with the 200 plus adaware type things and 3 virus Malwarebytes found. She had up to date avira.?