Outlook Express 6

We are trying to send a number of emails at once (50 approx) which are in a Group (address book)
when we send (email) the Group we get the following message :

The message could not be sent because one of the recipients was rejected by the server. The rejected e-mail address was 'harishk@sympatico.ca'. Subject 'New Quantegy FireWire Hard Disk Drive from Precision Sound Corp..', Account: 'pop51.bellnet.ca', Server: 'smtp10.bellnet.ca', Protocol: SMTP, Server Response: '550 Invalid recipient: <harishk@sympatico.ca>', Port: 25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC79

I realize if we removed the email address shown, we might be able to send the rest of the Group at one time., but for some reason the whole email transmission is stopped.

I thought with OutLook Express 6, that if one email was rejected or didn't go through, the rest of the emails in the Group would continue to be sent thru.
we are using windows xp

any ideas what we are doing wrong

thanks,joan