There's nothing clever here. You have the option on both Yahoo and Hotmail (and indeed most of the other major mail services) to set a reply to address - so the mail is sent out from yahoo etc. but in the reply to field, the user will see your proper POP3 mail and not
jaiem@hotmail.com which does not look good to your customers !!
However, some mail clients/servers show that the mail originated from a Yahoo etc. origin so it is not a perfect solution.
However, it is great in emergencies. I tend to prefer the use of a laptop on the road with an offline Exchange folder which fully syncs back in the office PC email so I do not lose any history of email etc.
But, we still need full mail access via the web and the world is still lacking in this area. Webbox is getting close - shame it's unreliable.