It sounds like a DNS issue (if it doesn't resolve, the server isn't playing a part). Go to kloth.net and type in your domain name (without any subdomains) and select NS (nameserver) under the Query section. Hit lookup, and it will tell you where the DNS is stored for that domain.
You need to go to wherever that is and add subdomains (they make call them additional A records) for, at least, webmail.yourdomain.com. If you're lucky, they may even support adding wildcard DNS - add a subdomain of * and all subdomains not specifically mentioned will resolve to it.
To confirm it's a DNS issue, use kloth's A record lookup, or try to ping the subdomain - if it doesn't resolve (it should say PING domain.com (IP address) if it does), that's the issue. Don't worry too much if the PING times out as it could well be your hosting company blocking it.
EDIT: I just noticed you provided the domain in your question. I've checked and, as I thought, webmail.lsbc.co doesn't exist (** server can't find webmail.lsbc.co: NXDOMAIN). The DNS is hosted with 1and1 so you'll need to login to their control panel and add it there, if you have any issues they should be able to help.