Before Ipstenu, Andrea_R, and mrmist jump in with knee-jerk reactions ...
Yes, I have read what you each posted here - http://wordpress.org/support/topic/multisites-work-actual-subdomains-being-redirected?replies=19 - before you prematurely closed that thread.
I return to what none of you were understanding - i.e. -
To access webmail with a browser, you have to input the address as something like webmail.domain.tld - in this case (and on all servers) "webmail" is an http accessed subdomain (browser interprets it as http://webmail.domain.tld), and the default WPMS wildcard subdomains handler is hijacking calls to it and redirecting them to the WPMS 404 error page with an invitation to register a new account.
@ mrmist - I HAVE submitted bug reports on this - several times, with detailed information including everything you an ipstenu requested, and every time, someone deletes the bug reports from trac. The WP dev community is in denial about this issue.
@ Andrea_r - look, I don't want to end up in dispute with you over this - I admire what you and your partner do, too much, for that, but this is absolutely NOT a server side process that is banjaxed - Parallel (Plesk) and the hosts (UK's biggest cloud hosting operator) have worked constantly on this for six weeks now and cannot get it to work.
I repeat what I requested in the closed thread -
If someone has WPMS 3.0.1 working on UNIX (not Linux) with Plesk on Cloud Hosting, please can they post the how-to?
If you're not on Unix AND Plesk AND Cloud hosting, it's likely your solution will not work - Cloud Hosting is very different to regular shared and VPS hosting.
Please do not tell me to change hosts - every other webscript runs on this host, and WPMU 2.9.2 ran perfectly on them. Standalone WP 3.x runs perfectly on them. WPMS 3.x WITHOUT wildcard subdomains runs perfectly on them - but as soon as wildcard subdomains are enabled, something in the WPMS constants/functions/whatever are hijacking calls to ALL subdomains and redirecting to WP's 404 registration page (on all tested themes, with and without any plugins including caching plugins, and including on a twentyten barebones new install).
Sorry that last paragraph sounds like ranting, but I am upset - OK? Plus, I can think of no other way to state it clearly.
The subdomain hijacking even occurs to attempts within Plesk to go straight to a webmail-box from the Plesk mail management console, (obviously enough because Plesk puts it into an http window to login and display the mailbox panel).
The hosts and I have tried every browser and version we can get our hands on in case it's a browser trigger somewhere, but all browsers return the same problem - I even rooted around in the boxes in the rice barn to find my copy of Internet Explorer 4 - and that had even worse issues - wouldn't even load plesk correctly (too many curvy boxes) - LOL.
So please - I've tried EVERYTHING each of you have said or suggested, and the hosts and Plesk techs have been working on this since mid-July, and can't get it working. I've tried numerous times to file a bug report the correct way and they're always deleted within 12 hours - so please don't hide behind that instruction.
I do understand that a small percentage of people are facing this issue, so perhaps it's time to stop censoring / silencing them, and start looking for the commonalities, in order to resolve these issues? If everyone else is happy and has working installs, then perhaps it's time to look at the edge cases?