Ken, sorry, Im still not exactly sure what you want. By web based I usually think of something like Yahoo mail or gmail. Both of these I think can have multiple addresses forwarded to them, but I dont think that you can download mail, use it offline, and then reconnect to send the replies or whatever. For downloading and doing stuff offline, then connecting to send I think of not a web based product but a client like Eudora or Outlook. Am I making sense explaining this? Or am I confused? I seem to recall a while ago that you were using a Mac. They have a built in mail program that I think works fine with web based mail hosts. Also, Eudora is a great program for the Mac (I think there is a PC version as well).
I personally use Outlook on PCs and Entourage for work mail since we have an Exchange server. For personal I have both Yahoo (paid version - $20 per year) and Gmail. I dont download from these but use them online. My wife downloads her personal email to her Mac mail client from her Yahoo address. Again, she has the $20 a year paid one since that allows you to use it as a POP server.