You should give a few more details in your question if you want a detailed
answer. I can only hazard a guess on the basis of such sparse info.
Basically, with POP/SMTP clients you can retrieve email from any source, but
email must be SENT via the connection-provider's SMTP server. This is only
reasonable as the provider doesn't want unidentified people sending stuff
(most likely spam) through their equipment.
You need to:
A: find out the DNS name of the cable company's SMTP server. The website
will probably tell you this.
B: Put this into your email program's settings , in place of the previous
provider's setting. In Outlook Express you find this under
Tools>Accounts>Mail>Properties, and on the second tab, 'Servers.'