To find out if you've been blacklisted via your ISPs server, contact your ISP and make an inquiry directly with them over the phone or through email. (honestly I doubt this is it, but you never know.)

The server you use to send out email can either be from your ISP, a web-based email account, or if you are an advanced emailer, one you've created yourself using software.

To turn off email scanning in Norton:

http://service1.symantec.com/SUPPORT...01082921552806


To complete disable Norton AV (temporarily):

http://service1.symantec.com/SUPPORT.../1997121131456


Note: Only do that temporarily to test if the AV is messing with the sending of email.


Note2: It may be necessary to complete remove Norton.. but I would hold off on that until you've tried to disable the two ways noted above.


As for Thunderbird and Eudora, they are neither web-based email services. They are similar to Outlook/Outlook Express in that they hand functions of sending and receiving email amongst other things. I recently made the switch to Thunderbird and am thrilled with it.

About Thunderbird with download:
http://www.mozilla.com/en-US/thunderbird/

More about Eudora with download:
http://www.eudora.com/download/