What's wrong with fetchmail? Granted, it's not "as soon as it arrives", but you can run it every minute if you want.
(I also use unison to keep the mail directory synchronized between all my machines.)
Also, how many times have you spoken to someone on the phone for them to say "I've just sent you the details.... can you see it yet?"
Within seconds, really? I haven't encountered a single service like that.
> Also, how many times have you spoken to someone on the phone for them to say "I've just sent you the details.... can you see it yet?"
True but there's an easy solution: Fire up a terminal or even use a keyboard shortcut to run offlineimap manually within a split second.
I've had a variant of this due to using a custom domain with a small email provider that was either slow or greylisting; the second-factor login email for GOG took longer than 15 minutes to arrive, by which time it had expired. I had to keep trying until it came through quickly enough.