It was Google, which broke away from the rest and offered us a gigantic, never before heard of, 1000 megabytes (1 gigabyte) of free email storage with Gmail (or Google mail).
They were soon followed by Yahoo and later by Hotmail (Now Windows Live Hotmail).
However, whatever they did and how much they upgraded, Gmail was always one up. That is, until recently. Today I noticed that Yahoo! is offering UNLIMITED storage to both new and existing users. More information here.
Can't wait to see how Google and Microsoft are going to respond to that.