If the small ISP opens their SMTP server to the IP addresses of the big national dialup provider, which they would have to do in order to be able to handle that roaming customer who could be just about anywhere, will they not also be opening themselves up to being a relay for any spammer that uses any reseller of that national provider? Will not such spammers then have access to every ISP doing reselling via that national one?
How about checking the MAIL FROM: part of the message? If a spammer tries to use an invalid address, the small isp's sendmail can reject it outright.
I think the SMTP server that should be used when dialing that national provider is the SMTP server provided by that national provider, unless some kind of VPN is used (to be more technically correct, use the SMTP server of the provider of IP addressing).
Yes, but how does the user know to change this? A redirection (some policy routing and port fudging will help,) but it may be easier for the visp client to be authenticated in some way, (like a radius scheme that picks up the current remote ip address of the user and allows relay from that address while the user is online.) TERRY