If you mean which mail client, I use the Opera mail client, though I also have used Thunderbird, Kmail, Evolution, and Sylpheed, but I keep coming back to Opera.
Evolution is less than desirable. Sylpheed is excellent. I like the Kmail interface, but I don't like that it fires up the Akonadi server thingee in the background. I find Thunderbird heavy.
If you mean ISP, Cox Cable here in the US. Their online support isn't very good; they still have tutorials for Outlook Express, for Pete's sake. Comcast, who I used to have before I moved, had excellent online help pages, including for several Linux mail clients.