Make sure you are typing in www.[YOURDOMAIN].[TLD]/webmail/, so www.example.com/webmail could be one, as could example.org/webmail/

It's also important that you use the full email to log in. "john" will not work, but "johndoe@example.com" will work.

Last, but not least, try to log in at our alternate location www.hostarmor.com/webmail/ that should get you into the shared email server.