sub domains

[kevin]

We have several domains hosted and all work OK
but when we try to add a sub domain eg, sub.domain.com
the mail bounces with an error message

Sorry, no mailbox here by that name

If a mail is sent to [EMAIL PROTECTED] it works
but [EMAIL PROTECTED] does not

in /var/qmail/control/virtualdomains is

sub.domain.com:username

What is wrong here

Kind regards
Kevin