Cannot send emails using my work domain !

I've been using Outlook for years now as my email cilent for ALL mail accounts viz:

1. My work domain mails
2. My personal domain mails
3. Gmail
4. Hotmail

The system has been working fine all this while. Recently I switched to a new laptop and obviously installed MS Office 2003 applications.

For the last week or two I've been having a problem in sending out emails from the work and personal domain accounts.  The details are as below:

1. When the problem first started, it would just not send the mails out from any account. The mails would sit in the outbox (un-italisised) and not move. No error messages here.
2. Here I had to constantly reopen the messages, save them to my DRAFTS folder and then resend them. After doing this process over 50 times I checked the online help for Outlook.
3. This seemed to be a problem related to ADD-INS as per the site. The add-ins identified that related to my installed programmes were Norton Internet Security (this came preinstalled on my laptop).  The solution was to disable this particular add-in to be able to resolve the issue.

Now the developments are as follows :

1. The Norton add-in won't get disabled in Outlook.
2. I disabled the Norton anti spam feature from Norton Internet Security.  But it sill wont disable the add-in.
3. Now I can send emails only via the GMAIL and Hotmail accounts.
4. My work and personal domain mails give out the following error message :


Task 'AuroRA - Sending' reported error (0x800CCC78) :
'Unable to send the message. Please verify the e-mail address in your account properties.  
The server responded: 553 5.3.0 Spam Detected  http://list.dsbl.org 220.224.41.149'

Now one option was that it could be an ISP problem. However I tried using different dial up connections and the same issue on all.
Furthermore, the rest of the staff in my office has no difficulting using the work domains from their Outlook systems!

I deleted both these accounts and re-added them. But it wont work.   I also tried creating the particular email accounts in Outlook Express but that wont work either. It gives me teh same error.

I've also checked back with my hosting co.  I can send mails using the webmail options for the domains I use. So this problem seems to be with settings on my machine.

So I've concluded this is a setting issue on my machine but I can't figure out what setting is causing this issue. If it is related to the Norton Internet Security feature, is it ok to uninstal Norton and use AVG's free version or something else in its place or can I continue using Norton and work on the settings.

I've put in the maximum details above on the issue. Hope this helps the expert in understanding the nature and helping me fix this.

Thanks in advance