Any webserver anywhere exists for the purpose of serving files to visitors (webpages, images, sounds, etc.). Each item served is of a certain size, measured in kilobytes.
A webserver is only capable of sending out a certain amount of information per unit of time, e.g. gigabytes per month.
All webservers cost someone a pretty large chunk of money to keep up and running, and the Internet connection for the server is charged for (or the connection's "size", or bandwidh, is determined) by the number of gigabytes per month served.
There are two kinds of webspace providers on the Web, available to host pages for people who don't own their own webservers–free, and pay.
Free webspace is given away because the provider places advertising on the pages visitors see on their server–the advertising pays for the bandwidth, so the service is free to the person building a webpage there. Free webspace providers have rules, Terms of Service (or some similar name), which anyone signing on to build a page agrees to abide by. One of these rules requires that all files on the site be used on the server, not simply for storage and remote serving (making the content available in a form not on the free-server pages). This is because the free server can't make money if their service resources are used without their ads being visible.
Pay providers really couldn't care less whether their paying subscribers use their paid account's resources onsite, or off site. Why? Because users of paid servers pay for the bandwidth themselves (or are limited by the month)–that means if their pages use up the allotted bandwidth for the price they pay, they will either get hit with a big extra fee, or their pages will be made unavailable till the next billing cycle (the next month).
This is why Tripod has placed a block on serving files to remote locations (such as another provider's webpages, or in e-mail or newsgroup posts), it's costing them money and they get nothing in return (not even a mention of the Tripod name). If any WebTVer wants the unfettered ability to put anything in their mail sig without worrying about being cut off, they would have to get paid server space, and pay for sufficient bandwidth to support their signature "habit."
HTML mail is fun, but remember that if you are loading images and sounds from a free webspace provider you are in technical violation of Terms you have previously agreed to. If you get "caught," they got ya, and you have no basis for complaint or redress.
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