1) Take a look here: http://www.faqforge.com/linux/contro...-his-password/
2) You mix up email account limits and traffic quota. There is a limit for the number of email accounts and the storage space of course but there is no limit for the amount of traffic per email account. At least in germany webhosting companys limit email accounts only by the number of accounts and the number of storage space but not be the traffic anymore.
@ben: There is no such interface until someone will write it.