email bounces with "Delivery failed" email "Unknown user".

Are you using a self hosted Exchange/other e-mail server enviroment? Or are you going out via an ISP? are users at the same domain going through while others are bouncing?

if you are using pop3 email, try to enable the SMTP Authentication for you email

if you are using Microsoft Outlook then:
Open Outlook
From the Tools menu, choose E-mail Accounts
Click the View or change existing e-mail accounts radio button in the E-mail area.
Click the Next button
Choose the email account you want to configure SMTP authentication for from the list.
Click the Change button.
Click the More Settings button
Click on the Outgoing Server tab.
Check the box next to My outgoing server (SMTP) requires authentication
and choose "use same settings as my incoming mail server"

if you are using outlook express then :
Once Outlook Express starts click the “Tools” menu at the top of the window and then click “Accounts”.
click on the “Mail” tab at the top of the window. You should see your Dnet email account listed in the box below. Click on its name to highlight it and then click the “Properties” button on the right side of the window then lick on the servers tab at the top.
At the bottom of that window put the check in the box that says “My server requires authentication”. Then click the "settings” button to the right.
then chjoose "use same settings as my incoming mail server"

Unfortunately that is not the solution, I already have  that specified.  The mail  does go out but SELECTIVE email address work and many do not.

JBM, by selective do you mean you can e-mail person X@domain.com but not person Y@domain.com, or is it specific to certain domains?

it seems that john@xxx.com or mary@xxx.com will fail  but @ can send to josh@gmail.com

It sounds almost as if for some reason those domains may be filtering your mail. Are you using e-mail via your own mail server, via an ISP, or what? I am trying to remember the name of the appropriate registration, but some spam filters such as Ironport will assume you are spam if your mail server is not registered properly. I'll see if I can find the record that needs to be placed if it's a mail server issue. If you are not using your own mail server, this is a moot point however.

I assume their main contact info works if you use a different address (ie a gmail account)?

If you have an alternate address that works, and you can't send e-mail to anyone in their domains, i would defineatly say that you are being blocked for some reason by those specific domains. If you are going through an ISP you may want to research if that ISp is being considered a threat for spam. Some filtering applications will generically block from "bad reputation" hosts as well.

The problem occurs when I do not use verizon (one of my ISPs a line provider)  as the outbound SMTP server.
my inbound POP ISP refers to my domain and email host.

It now works fine

for some reason the same test did NOT work with comcast as my outgoing smtp (I have both)

Not all ISP's will reject mail relay based on sending e-mail address, it is quite common though. Some will allow relay with any from address, as long as IP address od sender is local to server. That is probably why you do not have any trouble with comcast.