I recently switched from an sbcglobal ISP provided address (uses Y! mail) to a plain Yahoo address. Luckily, since the ISP address gets POP3, I could pop my Emails by the hundreds, maintaining the original message header info (time, from…) and manually sorting massive quantities into the proper folders. It took a little while and I often had to reload the page because POPing several hundred messages into Y!MB at once usually crashed it. After refreshing, I would usually have maybe a third or half of what I was POPing, so then I’d just POP the rest. I even got the dreaded Error Code 5 a couple times, but then I just signed out/in and Y!MB worked fine. Y!MB was VERY helpful for this because it let me scroll through folders’ entire contents, so I didn’t have to constantly hit NEXT to find where a partial POP left off. Y!MB even lets you drag messages in and out of the sent folder (possibly partially by my request) so I even synced sent messages. Now I have a fully synchronized Email account, and w/ the Y! Photos to Flickr transition, I was able to sync my pics over to my @Yahoo.com email address as well

In the Hotmail>Live situation, this couldn’t be done unless they paid for POP. Otherwise they’d have to forward Emails one by one, taking hours and breaking from and date info.

I think web based mail should have some easy sync method. You should be able to transfer Emails and contacts from one provider to another, like you can with desktop mail. And it should always be reversible. If Webmail was more like this, people would be less likely to use the same Email site their whole life, thus encouraging competition.