Simple SMTP/network routing questions

At home my connection is with comcast and I have my clients configured to send via whether they're for my comcast account or (file hosting, mainly) domain or domain.Now, I use OE for my comcast account and full Outlook for the other two. Outlook doesn't have my Comcast authentication information anywhere but the mail still goes through. However, from work, stuff wont send out on I have a few questions and you're welcome to tell me anything else I might want to know, you see as related, or... whatever.
1) Is it basically that I can't get to Comcast's smtp server through the public internet? The only tables that know where the SMTP server is are on the provider edge where the customer comes in?
2) Why does mail have to come from an SMTP server? And how does the "@" figure in to that?

Please help.

I didn't find the right solution from the Internet.

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