lookout express? i have phoned tech support once in my life. They took 15 minutes to go through all the stuff i had already checked, then rung off, phoning me back, ringing me back to tell me it was a problem at their end (this is for my then ISP)
Yes, i have had this problem with a couple of sites when using Opera, too. As you say, identifying as something else doesn't help. Oddly enough this exact same problem does occur with the tomgreen.com email service, which is run by bigmailbox.com (according to netcraft they are running Apache/1.3.6 (Unix) mod_perl/1.21 on Linux) using windows with mozilla 1.0... odd, also odd, that it does not happen when using windows with Opera 6, identifying as mozilla.
So is it really OS paranoia? (it might be, bigmailbox might be paranoid against people who don't use linux!) or is it something altogether more boring? (like some crappy javascript incompatibility for example...)?